Manage pending in memory and move it to synchroniser

[IronGauntlets]

Fixes ~parts of~ #1789. ~A migration to remove the pending from DB is still outstanding.~

It would be best to review the commits individually and in order.

[IronGauntlets]

This also fixes #1450

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 68.99225% with 40 lines in your changes missing coverage. Please review.

Project coverage is 75.36%. Comparing base (5122970) to head (66f53e1).

Files	Patch %	Lines
rpc/transaction.go	52.50%	14 Missing and 5 partials :warning:
sync/sync.go	63.46%	15 Missing and 4 partials :warning:
blockchain/event_filter.go	85.71%	0 Missing and 1 partial :warning:
rpc/helpers.go	90.90%	1 Missing :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1844      +/-   ##
==========================================
- Coverage   75.40%   75.36%   -0.04%     
==========================================
  Files         100      100              
  Lines        8982     8915      -67     
==========================================
- Hits         6773     6719      -54     
+ Misses       1604     1600       -4     
+ Partials      605      596       -9     

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[IronGauntlets]

Since I will be away for 2 weeks. Please "Rebase and Merge" once it is approved.