Long regtest chain synchronization

[RuslanProgrammer]

simplexl regtest command takes ~7s to launch. Where ~5s of this is taken up by Electrs indexing the only one tx (after generating 100 initial empty blocks).

This 5s value is not random - it is the frequency of the main loop iterations.

Expected behavior:
The startup process should take 5 seconds or less for the described scenario.
The main loop iteration should be executed after new tx in the mempool, and not any ms before.