MB-12339 Make FDB_MAX_KEYLEN, FDB_MAX_METALEN, FDB_MAX_BODYLEN public

Note that we plan to support a longer key that is greater than the current
max key length 3840 bytes.
(http://www.couchbase.com/issues/browse/MB-11766)

Change-Id: I6e005b87cc16515c9df3821b32536362930d505f