log: Use severity-based logging for leveldb/libevent messages, reverse LogPrintLevel order

Summary:
Log messages from leveldb and libevent libraries in the severity+level based log format introduced in core#24464.

logging: Unconditionally log levels >= WARN

Messages with level WARN or higher should be logged even when
the category is not provided with -debug=, to make sure important
warnings are not lost.

https://github.com/bitcoin/bitcoin/pull/25202/commits/bd971bffb02c7b06aac9a479f7e5ed8f71de2bec

http: Use severity-based logging for messages from libevent

Map libevent's severity to our own severity level for logging.

https://github.com/bitcoin/bitcoin/pull/25202/commits/18ec120bb9e1fc9d27d2419da4c580bd3cde7e86

leveldb: Log messages from leveldb with category and debug level

https://github.com/bitcoin/bitcoin/pull/25202/commits/ce920713bf0810614c2c0c994511b50d4f660bce

refactor: Change LogPrintLevel order to category, severity

This is more consistent with the other functions, as well as with the
logging output itself. If we want to make this change, we should do it
before it's all over the place.

https://github.com/bitcoin/bitcoin/pull/25202/commits/c4e77177276ea2b79c4675cb2678ee2cc757b743

This is a backport of core#25202

Depends on D17217

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D17222