Add a retry register for sqlite3.BUSY

[nyankers]

It may be favorable to retry endlessly when a function returns sqlite3.BUSY, as that's basically what sqlite expects you do to.

This can't be done in a transaction though, boo.

We may want sqlite3.BUSY to fail sometimes, so maybe add a register that controls this, e.g. em.retry

Thus it'll only retry if transaction == nil and em.retry.

[nyankers]

Possibly implemented by fcfd034 but untested.