A SQLiteBatch need to wait for any other batch to finish writing before
it can begin writing, otherwise db txn state may be incorrectly
modified. To enforce this, each SQLiteDatabase has a semaphore which
acts as a lock and is acquired by a batch when it begins a write, erase,
or a transaction, and is released by it when it is done.
To avoid deadlocking on itself for writing during a transaction,
SQLiteBatch also keeps track of whether it has begun a transaction.
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