* Add stats and counter around cost model ops, mainly:
- calculate transaction cost
- check transaction can fit in a block
- update block cost tracker after transactions are added to block
- replay_stage to update/insert execution cost to table
* Change mutex on cost_tracker to RwLock
* removed cloning cost_tracker for local use, as the metrics show clone is very expensive.
* acquire and hold locks for block of TXs, instead of acquire and release per transaction;
* remove redundant would_fit check from cost_tracker update execution path
* refactor cost checking with less frequent lock acquiring
* avoid many Transaction_cost heap allocation when calculate cost, which
is in the hot path - executed per transaction.
* create hashmap with new_capacity to reduce runtime heap realloc.
* code review changes: categorize stats, replace explicit drop calls, concisely initiate to default
* address potential deadlock by acquiring locks one at time
* replay stage feeds back realtime per-program execution cost to cost model;
* program cost execution table is initialized into empty table, no longer populated with hardcoded numbers;
* changed cost unit to microsecond, using value collected from mainnet;
* add ExecuteCostTable with fixed capacity for security concern, when its limit is reached, programs with old age AND less occurrence will be pushed out to make room for new programs.
