This work aims to improve the structure and performance of the wallet's Sapling increment witnesses flow.

The process is essentially walking through the entire wallet's txs map a minimum of 2 times, if there is no shield tx in the arriving block, and adds one more for each shield tx inside the block. Which affects mainly to wallets with a large number of transactions as the wallet is locked while this operation is being executed.

So, straight to the point, the current increment witness workflow is:

1) for-each tx in wallet : call CopyPreviousWitnesses.
2) for-each tx in block:
   a) for-each note in tx:
      a1) for-each tx in wallet : call AppendNoteCommitment.
      a2) if note is mine : call WitnessNoteIfMine.
3) for-each tx in wallet : call UpdateWitnessHeights.

And, have re-structured it into:

1) for-each tx in block : gather note commitments in vecComm.
2) for-each shield tx in wallet:
  a) copy the previous witness.
  b) for-each note commitment in vecComm:
      b1) append notes commitment.
      b2) witness note if ours.
  c) Update witness last processed height.

Finishing with only one loop over the wallet's txs map in the entire increment witnesses workflow.

PD: The benchmark that did is still a bit raw..