Refactor flushing of stats and logs at ccache exit

Refactor flushing of stats and logs at ccache exit

Perform flushing of statistics counters (potentially triggering cache
cleanup) and log files in a finalizer inside cache_compilation instead
of in Conxtext’s destructor. This moves somewhat complex logic from
Context into the main ccache code.

As a side effect of this, stats_flush and stats_flush_to_file are
superfluous.