feat: wait until executions are over before exiting

[mpetrun5]

Wait until pending executions are over until relayer is shut down.
This is done via a RWMutext.
Executors acquire RLock which is non blocking until the relayer terminate signal is triggered
when a write Lock is acquired which waits until the RLock are finished and will have priority when
the initial RLocks are done.

Description

Related Issue Or Context

Closes: #178

How Has This Been Tested? Testing details.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation

Checklist:

• I have commented my code, particularly in hard-to-understand areas.
• I have ensured that all acceptance criteria (or expected behavior) from issue are met
• I have updated the documentation locally and in docs.
• I have added tests to cover my changes.
• I have ensured that all the checks are passing and green, I've signed the CLA bot

[github-actions]

Go Test coverage is 66.7 %\ ✨ ✨ ✨

[github-actions]

Go Test coverage is 66.7 %\ ✨ ✨ ✨