Use gcc attribute to force spinlock padding to 64B.
Move most of mutex.h into hardware.h.
Do WT_CACHE_LINE_ALIGNMENT for gcc spinlocks as well.
Implement Alex's suggested "are we evicting" check: don't look for
obsolete WT_UPDATE structures unless we have eviction candidates.
Remove a warning about const-ness from hash_fnv.c, minor cleanup, no real change.
Fix compiler warning:

../../../bench/wtperf/wtperf.c: In function ‘execute_populate’:
../../../bench/wtperf/wtperf.c:721: warning: implicit conversion
shortens 64-bit value into a 32-bit value
Add explicit active state to eviction server.

Used to limit when we clean up obsolete update lists.
Merge branch 'develop' into fine-grained-durability
Fix some compiler warnings.

Printing off_t seems a bit fraught. It is usually a signed value, so
casting to intmax_t is the recommended solution.