bufferlist: replace Mutex with RWlock

bufferlist: replace Mutex with RWlock

Replace Mutex with RWlock in bufferlist code, so it doesn't block on reads,
also make it a tiny bit smarter, so it doesn't block writes on empty crc map
clears. Use no-tracking RWlock for better overall performance across entire
codebase, and don't use RWLocker helper classes for even larger performance
gains.

Signed-off-by: Piotr Dałek <piotr.dalek@ts.fujitsu.com>