Persist subscription status across reconnects

[tnull]

It would be nice if the client would re-subscribe to block headers/scripts if the connection was ever dropped and we're reconnecting.

Alternatively, script_pop/block_headers_pop could return an error indicating that the subscription has been cancelled in the meantime.

[tnull]

Looking at the code I just realized we don't actually seem to reconnect in the first place, which would of course be a prerequisite for this (cf. #165).