Cache config value in LdapAuthBackend

Looking up a configuration value can be slow at scale, sometimes the
scan within JGit is O(N) and sometimes its O(log N). Either way is
slower than caching the setting at startup as a primitive boolean.
Gerrit coding style is to cache these sorts of configuration options.

Change-Id: Ifd6b84eda4ea11ffec5bed14000fffd3480fc704
1 file changed