Disable TransactionPool observer on new blocks during Full sync

[ahamlat]

Description

During Full sync (archive node), the transaction pool observer event is triggered for each new imported block to re-organize the transaction pool (remove block's transactions, sort one of the layers per base fee, ...etc).
Besu should disable transaction pool event during Full sync (and other syncs)

Steps to Reproduce (Performance issue)

Start Besu with Full sync and Forest.

Expected behavior:
Besu does not trigger the transaction pool observer event for each new block

Actual behavior: [What actually happens]
Besu triggers the transaction pool observer event for each new block

Frequency: [What percentage of the time does it occur?]
100%

[fab-10]

Some context to help with the implementation.

• TransactionPool now has methods setEnabled and setDisabled.
• The txpool should be disabled when the current local chain head is too far from the target chain head, SyncState::isInSync already provide this information

[macfarla]

@fab-10 are you working on this one?

[fab-10]

yes, currently on it