Move PinBuffer and UnpinBuffer to atomics

Hello hackers!

Continuing the theme: http://www.postgresql.org/message-id/3368228.mTSz6V0Jsq@dinodell

This time, we fairly rewrote  'refcount' and 'usage_count' to atomic in
PinBuffer and UnpinBuffer (but save lock for buffer flags in Unpin).

In the same time it doesn't affect to correctness of buffer manager
because that variables already have LWLock on top of them (for partition of
hashtable). If someone pinned buffer after the call StrategyGetBuffer we just
try again (in BufferAlloc).  Also in the code there is one more check before
deleting the old buffer, where changes can be rolled back. The other functions
where it is checked 'refcount' and 'usage_count' put exclusive locks.

Also stress test with 256 KB shared memory ended successfully.

Without patch we have 417523 TPS and with patch 965821 TPS for big x86 server.
All details here: https://gist.github.com/stalkerg/773a81b79a27b4d5d63f

Thank you.
--
YUriy Zhuravlev
Postgres Professional: http://www.postgrespro.com
The Russian Postgres Company