This set of changes removes the list of available buffers and instead
simply uses the clock-sweep algorithm to find and return an available
buffer.  While on the surface this appears to be removing an
optimization it is in fact eliminating code that induces overhead in the
form of synchronization that is problemmatic for multi-core systems.
This also removes the have_free_buffer() function and simply caps the
pg_autoprewarm process to at most NBuffers.

Change nextVictimBuffer to an atomic uint64 and simply atomically
increment it by 1 at each tick.  The next victim buffer is the the value
of nextVictimBuffer modulo the number of buffers (NBuffers).  The number
of complete passes of the clock-sweep hand is nextVictimBuffer divided
by NBuffers. Wrap-around of nextVictimBuffer would require 10 years at
~59 billion ticks per-second without restart, so unlikely that we ignore
that case entirely.

With the removal of the freelist and completePasses none of remaining
items in the BufferStrategyControl structure require strict coordination
and so it is possible to eliminate the buffer_strategy_lock as well.

Re-author the clock-sweep algorithm such that it maintains its own state
and has a well defined API.