Switch to spdlog macros

[Shillaker]

The aim here is to switch to using spdlog macros (SPDLOG_DEBUG, SPDLOG_INFO etc.) for two reasons:

• Replace our current logging implementation which is a point of contention in a multi-threaded environment.
• Allow us to noop trace and debug statements in release builds.

How:

• Global find and replace on all the logging statements.
• Set the SPDLOG_ACTIVE_LEVEL macro depending on whether it's a release build. This sets the minimum possible logging level (i.e. if it's set to info (2 in this case), this means the debug and trace macros are nooped).
• Reintroduce the initLogging function which calls spdlog::set_level to set the active logging level for that execution. This selects which log statements get printed out at runtime and uses the faabric::util::SystemConfig::logLevel property as before.

This means, in a debug build you can still change LOG_LEVEL to debug or trace without recompiling, because we have SPDLOG_ACTIVE_LEVEL set to 0, thus allowing all levels of logging. However, in a release build, even if you set LOG_LEVEL to debug, the debug statements won't get printed as the macros are nooped.

I've added the relevant comments in the code, including some warnings if you've chosen a level that won't get printed.

spdlog docs for reference.

The only files with non-trivial changes are:

• include/util/logging.h - sets SPDLOG_ACTIVE_LEVEL, i.e. the minimum possible level.
• src/util/logging.cpp - contains the implementation of initLogging which sets the current logging level and log format.

[csegarragonz]

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