lib ldb: Rename functions to ldb_kv

lib ldb: Rename functions to ldb_kv

Rename the ldb key value functions from ltdb_* to ldb_kv_*. The renaming
is preparation for the separation of the tdb specific code from the key
value code.  This work is a follow on from the addition of the lmdb
backend.

Note that the next commit tidies up the code formatting.

Signed-off-by: Gary Lockyer <gary@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>