Remove global symbols: Avoid using the global namespace if possible.

Partially resolves #15612 ("Reduce the number of global symbols used").

Change in global symbols as reported by `nm bitcoind` before vs after:

```
$ diff -u <(nm src/bitcoind-before | c++filt | grep -E '^[0-9a-f]+ [A-Z] ' | cut -f3- -d' ' | sort -u) \
          <(nm src/bitcoind-after  | c++filt | grep -E '^[0-9a-f]+ [A-Z] ' | cut -f3- -d' ' | sort -u) \
          | grep -E '^[+-][^+-]'
-boundSockets
-cs_warnings
-eventHTTP
-fFeeEstimatesInitialized
-fLargeWorkForkFound
-fLargeWorkInvalidChainFound
-pathHandlers
-strMiscWarning[abi:cxx11]
-threadHTTP
```

diff -u <(nm src/bitcoind-before) <(nm src/bitcoind-after) | less