More generic layout for caching.

- new $wgBugzillaCacheType, replaces dropped $wgBugzillaUseCache
  and $wgCacheObject
- new Bugzilla::getCacheClass() and ::getCache() static methods
- new BugzillaCacheI::setup() abstract method, called by BugzillaCreateCache()
- new BugzillaCacheSql class, handling any generic MW SQL-backend;
  BugzillaCacheMysql now derives from it (but could be dropped,
  there's not much specific to MySQL here);
- separate SQL CREATE TABLE definitions for MySQL, PostgreSQL and SQLite.
- base64_encode/_decode() cached values (prevents issues about escaping data for SQL storage)