Switch off gossiped txs during block production

[benaadams]

Changes

• Don't accept new tx for pool between Block processing starting and Forkchoice; is additional load on the State db and unknown if the txs will be valid after Forkchoice
• Also removes contention between the Block producer removing completed tx from pool and new txs being added

Types of changes

What types of changes does your code introduce?

• Optimization

Testing

Requires testing

• Yes
• No

If yes, did you write tests?

• Yes
• No

Notes on testing

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Documentation

Requires documentation update

• Yes
• No

If yes, link the PR to the docs update or the issue with the details labeled docs. Remove if not applicable.

Requires explanation in Release Notes

• Yes
• No

If yes, fill in the details here. Remove if not applicable.

Remarks

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[LukaszRozmej]

Not sure if this is good idea - lets have a discussion about it.