fix: remove unique index on peer_address

The peer_address parameter is an inbound address used for communication with the peer. It shouldn't be confused with the listen address. The persisted peer_address may become obsolete due to circumstances beyond picodata control (e.g. DNS or IP changes). Thus there's no point in its prior validation, including the uniqueness check.

There's also no such task as getting peer by peer_address.

To sum up, an index over peer_address is useless. It only creates problems and causes panics.

Close #88 (closed)