Severity-based logging, step 2

Summary:
user-configurable, per-category severity log levels based on an idea by John Newbery and refined in GitHub discussions by Wladimir Van der Laan and Marco Falke.

• simplify the BCLog::Level enum class and the LogLevelToStr() function and add documentation
• update the logging logic to filter logs by log level both globally and per-category
• add a hidden -loglevel help-debug config option to allow testing setting the global or per-category severity level on startup for logging categories enabled with the -debug configuration option or the logging RPC (Klement Tan)
• add a trace log severity level selectable by the user; the plan is for the current debug messages to become trace, LogPrint ones to become debug, and LogPrintf ones to become info, warning, or error

Simplify BCLog::Level enum class and LogLevelToStr() function

• simplify the BCLog::Level enum class (and future changes to it) by only setting the value of the first enumerator

• move the BCLog::Level:None enumerator to the end of the BCLog::Level enum class and LogLevelToStr() member function, as the None enumerator is only used internally, and by being the highest BCLog::Level value it can be used to iterate over the enumerators

• replace the unused BCLog::Level:None string "none" with an empty string as the case will never be hit

• add documentation

(Note that core#29419 (D17217) deduplicated the category names.)

Add BCLog::Logger::m_log_level data member and getter/setter

Co-authored-by: "klementtan <klementtan@gmail.com>"

Add BCLog::Logger::m_category_log_levels data member and getter/setter

Co-authored-by: "klementtan <klementtan@gmail.com>"

Add BCLog::Logger::SetLogLevel()/SetCategoryLogLevel() for string inputs

and remove unnecessary param constness in LogPrintStr()

Co-authored-by: jonatack <jon@atack.com>

Update LogAcceptCategory() and unit tests with log severity levels

Co-authored-by: "Jon Atack <jon@atack.com>"

Create BCLog::Logger::LogLevelsString() helper function

Co-authored-by: "Jon Atack <jon@atack.com>"

Unit test coverage for log severity levels

Co-authored-by: "Jon Atack <jon@atack.com>"

Create -loglevel configuration option

• add a -loglevel=<level>|<category:level> config option to allow users to set a global -loglevel and category-specific log levels. LogPrintLevel messages with a higher severity level than -loglevel will not be printed in the debug log.

• for now, this config option is debug-only during the migration to severity-based logging

• update unit and functional tests

Co-authored-by: "Jon Atack <jon@atack.com>"

Unit test coverage for -loglevel configuration option

Co-authored-by: "Jon Atack <jon@atack.com>"

Note that the std namespace added for the << operator is needed so that BOOST_CHECK_EQUAL can digest enum class BCLog::Level).
It would normally be added in core#23497

Create BCLog::Level::Trace log severity level

for verbose log messages for development or debugging only, as bitcoind may run
more slowly, that are more granular/frequent than the Debug log level, i.e. for
very high-frequency, low-level messages to be logged distinctly from
higher-level, less-frequent debug logging that could still be usable in production.

An example would be to log higher-level peer events (connection, disconnection,
misbehavior, eviction) as Debug, versus Trace for low-level, high-volume p2p
messages in the BCLog::NET category. This will enable the user to log only the
former without the latter, in order to focus on high-level peer management events.

With respect to the name, "trace" is suggested as the most granular level
in resources like the following:

• https://sematext.com/blog/logging-levels
• https://howtodoinjava.com/log4j2/logging-levels

Update the test framework and add test coverage.

This is a backport of core#25614

Test Plan:
ninja all check-all

Add LogPrintLevel lines for each level in a function frequently called, start bitcoind -loglevel=... with the various levels (trace, debug, info) and check that only the expected messages are visible in the terminal.

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D18266