MB-100: Refactor CouchKVStore db file revision map

* initialize the db file map with default revision, 1
  this reduces unnecessary data directory scan and lookup
* use more efficient std::vector instead of std::map
* removed getFileNameMap/getDbFile/dbFileRev util. functions
* streamlined discoverDbFiles/populateFileNameMap
* renamed dbFileMap to dbFileRevMap to better reflect what
  the internal data structure really is for

Change-Id: I10eebca0cea959ca90282c2e704eeb7a91bd2c09
Reviewed-on: http://review.couchbase.org/23519
Reviewed-by: Michael Wiederhold <mike@couchbase.com>
Tested-by: Jin Lim <jin@couchbase.com>