Optimize membuffer cache index layout.  Groups should have a power
of two size to make them align properly with memory pages etc.
As part of this, factor out a group header struct which will be
extended soon.

* subversion/libsvn_subr/cache-membuffer.c
  (entry_t): 32 bits are enough for cache item sizes
  (group_header_t): new struct factored out from entry_group_t
  (GROUP_BLOCK_SIZE): new constant
  (GROUP_SIZE): move down and define depending on GROUP_BLOCK_SIZE
  (entry_group_t): use header and padding
  (drop_entry,
   insert_entry,
   initialize_group,
   find_entry,
   svn_membuffer_get_segment_info): update entry_group_t users

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1519265 13f79535-47bb-0310-9956-ffa450edef68