Make mutex impl configurable, use reentrant Monitor by default

Non-reentrant mutexes is a major PITA and may affect code that
uses Bunny.

Avg. publishing throughput with Monitor drops slightly from
5.73 Khz to 5.49 Khz (about 4% decrease), which is reasonable
for Bunny.

Apps that need these 4% can configure what mutex implementation
is used on per-connection basis.
Change log update
0.9.7
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