Make the server work with custom initial issuance prevout (liquid feature)

[enaples]

In order to do not rely on liquid testnet faucets and do not wait for pegin/out time of liquid test network to test mobile/desktop application, it would be awesome to index custom elements sidechains. In order to do that, the hard-coded regtest/testnet issuance prevout must be overcame and be able to set via CLI option.

Possible related issue: #125

[philippem]

please rebase

[shesek]

I don't like the global RwLock, it adds overhead in a hot path and won't work well with multiple electrs instances running in the same process (like we have in the tests). But explicitly forwarding the config option into has_prevout() would be kind of messy too.

I wonder, perhaps it would be cleaner to handle the initial issuance output using a dummy zero-value txo db entry instead?

Also, the CLI parameter is called --initial-issuance-prevout but it actually takes a txid rather than an outpoint. I would rename it to --initial-issuance-txid. (this does follow the INITIAL_ISSUANCE_PREVOUT naming which I originally chose, but we should probably change that too and at least not let it leak into user-visible options.)