Rewrite Task of listening Stellar Messages

Summary

We need to fix the vaults from getting unknowingly stuck, potentially caused by running unhandled zombie tasks.

One idea is to include the polling of the stellar messages: https://github.com/pendulum-chain/spacewalk/blob/7c7989875e95e1cfe3b0aeba9bb6af01d9e33e58/clients/vault/src/oracle/agent.rs#L100-L104 in the monitoring: https://github.com/pendulum-chain/spacewalk/blob/7c7989875e95e1cfe3b0aeba9bb6af01d9e33e58/clients/vault/src/system.rs#L806-L817

This means updating the OracleAgent's messagesender is delayed; passing the OracleAgent to tasks must be mutable; hence using **Arc<RwLock<>>_** instead of Arc<> alone.

But we cannot have these current tasks STARTING TOGETHER WITH the polling task. The Stellar-overlay has to run already, and all open requests MUST finish first. https://github.com/pendulum-chain/spacewalk/blob/7c7989875e95e1cfe3b0aeba9bb6af01d9e33e58/clients/vault/src/system.rs#L787-L790

An idea is to introduce another variant of the ServiceTask, where it waits for something to finish before a task starts. Prechecking will be required.

enum ServiceTask {
    ...
    // Runs a task after a prequisite check has passed.
    PrecheckRequired(Task),
}

And to make sure the stellar-overlay and the client are communicating well, stellar-overlay will also send to client the:

hello message - useful to signal the client to prepare itself
lib-related error - this way the client will not wait forever, if stellar-overlay has issues.

How to start reviewing:

`stellar-relay-lib`

Reduce clones in any way possible
- use pass-by-value instead, to avoid cloning references
  - from (&param) to (param)
- reuse variables that were passed by reference
  - made obvious in message_reader.rs file
- clone only the needed field, instead of the whole structure
Return a lib-specific error and hello message
- To signal the user (the agent) if the relay has started; and if the lib itself encountered a problem
Reposition timeouts and yields wisely
Let the user do the polling/looping

`vault`

Removed err-derive dependency, and just use the existing thiserror dependency.
Removed unused errors, like the RpcError
tokio_spawn for naming tasks. This is only viable for tokio_unstable.
Increase the event buffer capacity of the console-subscriber
OracleAgent adjustment
- Instead of Arc<OracleAgent>, it will be Arc<RwLock<OracleAgent>>
  - initializing the field message_sender is moved a bit later in the code; hence we need OracleAgent to be mutable.
Listening for stellar messages will be monitored, alongside other tasks.
- Every minute, a health check will be logged, to ensure messages coming from Overlay
- When receiving hello message, flag the proof building.
  - Continue to log messages, until the first slot was saved. Afterwards, this flag is not needed anymore.
proof_builder.rs
- Build proof ONLY if it's ready (if a slot was saved, meaning the overlay is running)
system.rs
- Added another variant of ServiceTask; the PrecheckRequired
  - Executing open requests is required before running some tasks; once finished, these tasks will start. The following tasks need this precheck:
    - issue::listen_for_issue_requests
    - issue::listen_for_issue_cancels
    - issue::listen_for_executed_issues
    - CancellationScheduler for Issue
    - CancellationScheduler for Replace
    - listen_for_replace_requests
    - listen_for_accept_replace
    - active_block_listener

I will add relevant comments when necessary.

pendulum-chain / spacewalk