* header MMR in use within txhashset itself
works with fast sync
not yet in place for initial header sync
* add the (currently unused) sync_head mmr
* use sync MMR during fast sync
rebuild header MMR after we validate full txhashset after download
* support missing header MMR (rebuild as necessary) for legacy nodes
* rename to HashOnly
* cleanup backend.append()
* simplify vec_backend to match simpler append api
* Include commitments non-duplicate checks in aggregate
* Remove said check from the pool
* Block building now uses tx aggregation to reduce duplication
* fix misbehaving simulnet fastsync test
cleanup redundant cutoff vs bitmap params is rewind and check_compact
* make sure we do not verify full kernel history on a writeable txhashset extension
rework simulnet simulate_fast_sync test to be more robust
* fixup store tests
* sleep for a bit longer to give nodes time to update
their sync_state correctly
* tweak timing of simulate_block_propagation
* Add kernel commitments sum and kernel and output MMR sizes to block header
* Sum a block without including previous sums, cleanup. Blocks are now summed and validated based on their own totals and not the totals since genesis. This allows to get rid of BlockSum and simplified the setting of a new block's roots, kernel sum and MMR sizes. Fixes#116
* Additional kernel MMR validation to check all prior header roots successively
* Wallet tests fix
