The dictionaries and proximities are not thread-safe. In order to
be able to check spelling in parallel, make a dictionary pool to
call upon when a spelling check is necessary.
Bug: 5156851
Change-Id: Ie3796164187dd7b7abf5ccd5d014073d43d74408
Until the dictionary itself is re-entrant, unfortunately we have
to synchronize accesses to it.
Bug: 5156851
Change-Id: I1629a160d707e6fa1ad80cba828798abb24b68b1
