LogPrint(category...) fixes

Two commits, two fixes.

First is an obscure infinite loop on Windows (and possible crash-at-shutdown). Most of the difference in that commit are just an indentation change from switching from LOCK(cs_...) to using mutexDebugLock.

Second is a fix for an obscure crash-at-shutdown on all platforms.

Tested by running without -debug, with -debug and with    -debug=net -debug=mempool.