Allow users to have multiple wallets in bitcoind

[araspitzu]

In this PR we force the usage of the default bitcoind wallet (named ""). This is necessary to allow the user to have multiple bitcoind wallets because when there are more than one we need to specify which wallet to use in the RPC calls. Fixes #1330.

Note for reviewers: the extra path in the bitcoin RPC URL identifies the wallet to be used and can be used in all the others RPC too. By default bitcoind has an unnamed wallet "" and eclair always expect to have that, users can add new wallets but should never rename or remove the default wallet (maybe we can add a few lines in the README for this).

[sstone]

I was able to reproduce #1330 on testnet and check that this PR fixes it, but as mentioned it's worth adding a few lines to our documentation.

[t-bast]

Open question: if we do that, why don't we allow the user to specify which wallet to use?
They could create a wallet for lightning, provide its name in eclair.conf and we'd use that.
And by default if they don't provide anything, we'd use the unnamed one.

[araspitzu]

Open question: if we do that, why don't we allow the user to specify which wallet to use?
They could create a wallet for lightning, provide its name in eclair.conf and we'd use that.
And by default if they don't provide anything, we'd use the unnamed one.

Why not :) as long as we can provide additional features without damaging the 0-conf setup i think we can add it.

[araspitzu]

Note for reviewers: at commit 44e009e this PR reads the bitcoin core wallet name from the configuration. Under some circumstances this introduces a new way the users can hurt themselves: between the fundTransaction and rollback operation we lock and unlock outpoints, if a user inadvertently restarts eclair before the rollback with a different walletname than it won't work and the outputs won't be unlocked. I think it's an acceptable risk (perhaps we can add a mention in the readme) because in general the users should not change bitcoind/wallet while there are pending operations in eclair.

[t-bast]

LGTM, but I haven't tested E2E (I'll let sstone re-do the test he did on testnet).

Under some circumstances this introduces a new way the users can hurt themselves

I think this is fine. Users that do that can't complain if bad things happen afterwards IMO, they have to know what they're doing.

[araspitzu]

On a second thought there are some other cases where the users can hurt themselves and even lose funds (think of swapping wallet and shutdown scripts usage), as a precaution i reverted the changes to implement only the original scope of this PR.

[sstone]

LGTM, just a wording change in our README.