Let the Child Chain server hint on its view of deposits recognition

[pdobacz]

A common scenario now is that:

1. Alice places a deposit
2. Alice's Watcher waits some finality and reports the deposit as a spendable UTXO
3. Alice wants to spend that UTXO but gets utxo_not_found

The main reason for this is a slower syncing of deposits on the Child Chain server end. Can take the form of:

• Child Chain server hiccups/stalls and is behind Alice's Watcher
• Child Chain server's Ethereum node is delayed wrt. Alice's for no big reason (the frustrated Pepe problem described by @InoMurko )

To gather the suggested solutions, that I've seen proposed:

• expose extra endpoints on the Child Chain server API to inform about its synced_height or directly its recognized deposits
• enable subscriptions on the Child Chain server API to allow it to push deposit recognized events to the Watcher
• enrich the utxo_not_found with details like probably_because_I_haven't_recognized_deposits_this_far
• the above combined with enriching the utxo_not_found with details like synced_height to give the caller an idea of where the Child Chain server is
• (quick, simple but incomplete patch) - split the deposit_finality_margin used in the Watcher into two: "state_processing_deposit_finality_margin" - drives OMG.State and deposit recognition for the use of syncing and "spendable_deposit_finality_margin" - drives the moment when Watcher (-Info) present deposited UTXOs as spendable. The latter is then > than the former and impact is partially minimized

Considerations:

• it must be stressed, that any response from the chch is informational and non-binding. Only valid inclusion of the submitted transaction in a valid, mined and confirmed block submission gives certainty, that the transaction has been indeed accepted
• solutions which "ask for permission not for forgiveness" are preferred, so as to not reveal transaction signatures prematurely (limit the transaction's in-flight span). However, note the above - the child chain server can always "lie" about the permission