Trim the transaction table

[MilkywayPirate]

Purpose

The TransactionTable and in particular non finalized transactions now only retains entries for accounts which have non-finalized transactions.
Accounts without non-finalized transactions are expunged from the transaction table and in that case the last finalized block is used for keeping track of the next available account nonce.

Changes

• When purging, then remove any entries (from the non-finalized transactions table) that has no non-finalized transactions.
• Don't populate the transaction table at startup.

Checklist

• My code follows the style of this project.
• The code compiles without warnings.
• I have performed a self-review of the changes.
• I have documented my code, in particular the intent of the
  hard-to-understand areas.
• (If necessary) I have updated the CHANGELOG.

[td202]

Mostly minor things. Otherwise, ensure that the account nonce is re-checked before adding incoming transactions (not from blocks), per our discussion.

[td202]

Good job!