Enable faster sync (#3108)

* add bitmap accumulator
refactor vec backend so we can use it outside of tests
introduce a "hash only" vec backend for the accumulator

* get core tests passing

* initial test coverage for bitmap_accumulator

* better test coverage for bitmap accumulator and cleanup code

* refactor txhashset roots, call validate() on roots during block validation

* fix store tests

* log the "merged" root when validating roots

* cleanup, revise based on feedback

* cleanup

* rework it to pass explicit size into bitmap accumulator when applying

api/src/types.rs

@@ -119,7 +119,7 @@ impl TxHashSet {
 	pub fn from_head(head: Arc<chain::Chain>) -> TxHashSet {
 		let roots = head.get_txhashset_roots();
 		TxHashSet {
-			output_root_hash: roots.output_root.to_hex(),
+			output_root_hash: roots.output_root().to_hex(),
 			range_proof_root_hash: roots.rproof_root.to_hex(),
 			kernel_root_hash: roots.kernel_root.to_hex(),
 		}