Decorate AuRaTxFilters in order to disable them post merge

[jmederosalvarado]

Changes:

• Disable all AuRa TxFilters post-merge in AuRaMergePlugin

Types of changes

What types of changes does your code introduce?

• Bugfix (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 not work as expected)
• Documentation Update
• Code style update (formatting, renaming)
• Refactoring (no functional changes, no api changes)
• Build related changes
• Other (please describe):

Testing

Requires testing

• Yes
• No

In case you checked yes, did you write tests??

• Yes
• No

Comments about testing, should you have some

Tested using posdao test setup

[LukaszRozmej]

I've simplified it a bit and fixed issue of fallback filter for MinGasPrice.
Is there a potential issue that we have a system transaction in TxPool already, that we should remove on PoS transition?