Fix exception when splitting F_sequences. (MinaProtocol#3199)

* Fix exception when splitting F_sequences.

Fixes MinaProtocol#3143.

* Changed split predicate from > to >= to match paper
* Fixed up other things to match
* Improved finger tree tests
* Fix handling of measures when splitting a subtree of Nodes and
  the split point is in the left finger.
* Added a transaction pool test for maintaining the max size.
  This would not have caught the bug, but is nice to have.

* fix warning