Documenting the (soon to be added) StatCacheCapacity directive.
Adding regression test for new StatCacheCapacity directive.
Use a slightly higher-than-default StatCacheCapacity for this test.
Major reworking of the backing store, using mmap(2) rather than SysV shared
memory segments.  Also fixed the locking so that it does byte-range locking
of just the relevant row of entries, rather than the entire table.

The capacity of the cache allocated is now configurable, via the new
StatCacheCapacity directive.