I don't love this, but it's the only way to select decoys without using a local
database. While the prior commit added such a databse, the performance of it
presumably wasn't viable, and while TODOs marked the needed improvements, it
was still messy with an immense scope re: any auditing.
The relevant scheduler functions now take `&self` (intentional, as all
mutations should be via the `&mut impl DbTxn` passed). The calls to `&self` are
expected to be completely deterministic (as usual).
The provided message-queue queue functions runs unti it succeeds. This means
sending to the message-queue will no longer potentially block for arbitrary
amount of times as sending messages is just writing them to a DB.
Moves the coordinator loop out of serai-bitcoin-processor, completing it.
Fixes a potential race condition in the message-queue regarding multiple
sockets sending messages at once.
Before we yield a block for scanning, we save all of the contained script
public keys. Then, when we want the address credited for creating an output,
we read the script public key of the spent output from the database.
Fixes#559.
