DatabasePool write barrier

[groue]

This pull request introduces a new DatabasePool database access method:

try dbPool.barrierWriteWithoutTransaction { db in
    ...
}

The barrier guarantees exclusive access to the database: the method blocks until all concurrent database accesses are completed, reads and writes, and postpones all other accesses until it completes.

There is a known limitation: reads performed by database snapshots are out of scope, and may run concurrently with the barrier.

The goal of the pull request is to provide support for users who need to change the passphrase of an SQLCipher database: we need a way to wait until all reads and writes are completed, rekey the database, and only then allow further database accesses. See #602.