Faster startup

[rustyrussell]

Don't require us to be fully synced with bitcoind before we allow certain operations:

1. Allow outgoing HTLCs immediately.
2. Wait for openchannel_init/openchannel_bump/fundchannel_start instead of failing.
3. Use bitcoind's height, not ours, for CLTV values on payments.

This means Greenlight doesn't have to block operations while waiting for block sync.

[rustyrussell]

Woops! There was already a test for this, I didn't need a new one. Will repurpose that instead!