This PR draft tries to fix issue #2409.

We don't store the announcement signatures before, so when restart we may waste much time on waiting for remote announcement reply. So this PR tries to store the remote announcement signatures into DB like what @SimonVrouwe proposed (we don't store local announcement signatures because we can drive sigs locally).

Instead of adding a new wire type between Channeld and MASTER, I change the wire_channel_got_funding_locked and ask it not only tell MASTER remote funding locked, but also send remote announcement sigs to MASTER and store sigs.

There are 4 changes:

1. Rename wire_channel_got_funding_locked to wire_channel_got_locked_and_announced, and add announcement data into the message of this type.

Now the message will be like this:
WIRE_CHANNEL_GOT_LOCKED_AND_ANNOUNCEMENT + got_remote_announcement(bool) + next_per_commit_point(pubkey) + remote_announcement_node_sigs(signature) + remote_announcement_bitcoin_sigs(signature)
The bool type above means if the message is to tell MASTER announcement or not(instead, to tell MASTER remote funding locked).

There 2 situations that this message will be sent:

• When Channeld receive remote peer funding_locked message, it will tell MASTER funding got locked remote. We will set the message like this:
  msg: WIRE_CHANNEL_GOT_LOCKED_AND_ANNOUNCEMENT + false + next_per_commit_point(pubkey) + NULL + NULL
  In this case, the msg equals the original wire_channel_got_funding_locked.
• When Channeld receive remote peer announcement message, it will tell MASTER the announcement signatures. We will the set message like this:
  msg: WIRE_CHANNEL_GOT_LOCKED_AND_ANNOUNCEMENT + true + NULL + remote_announcement_node_sigs + remote_announcement_bitcoin_sigs
  Channeld will check if we received the same announcement before and ignore the same announcement.
  Channeld only send wire_channel_got_locked_and_announced for announcement when it receives announcement for the first time.

2. Add announcement struct in channel struct (in lightningd/channeld.h).

struct announcement {
	secp256k1_ecdsa_signature announcement_node_sigs;
	secp256k1_ecdsa_signature announcement_bitcoin_sigs;
};

When MASTER receive announcement signatures, we will put signatures in this struct and store it into DB.

3. Add announcement table in DB, and add wallet_announcement_save() and
wallet_announcement_load() function.

• announcement table:
  The announcement table store data with the type like: (channelid, remote_node_sigs, remote_bitcoin_sigs).
  When we create a new channel and insert channel into DB, the announcement data will be set. If channel doesn't set ANNOUNCEMENT flag or hasn't received remote announcement message, we will set (channelid, NULL, NULL) for this channel.
• wallet_announcement_save():
  This function will only be called when MASTER receive wire_channel_got_locked_and_announced with flag setting true and resolve announcement signatures into announcement struct.
• wallet_announcement_load():
  This function will only be called when we initial a channel from DB (this will happen when restart).

4. Add announcement save and load test in wallet/test/run_wallet.c.

Compared to adding a new announcement wire type between Channeld and Lightningd, this way isn't so direct.
Anyway, it's just a try. What do you think about?