SQLite3 WAL mode

[Arantor]

The SQLite3 cache backend probably should set write-ahead logging mode so that other connections to the SQLite database can read rather than having it exclusive to connections.

Just after the call to busyTimeout(), there should be a call to $this->cacheDB->exec('PRAGMA journal_mode = wal;'); after.

More info: https://www.sqlite.org/wal.html

That said if the system cachedir is a network share this is still going to fail anyway with all the latency and pain that comes with it.