Improve performance of some RPC ledger commands using visitors.
Adds SHAMap::visitLeaves and Ledger::visitStateItems
Refactor net buffers classes
Add Request and Response HTTP parsers
Alphabet class for base58 conversions, Validators work
Fix Peers to stop after children are stopped
Validators work
Remove Journal from most Stoppable overrides
Validators work
Rename to key_equal for conformance with std containers
Validators work
Add RecursiveMutex, UnlockGuard, TryLockGuard
Add CPUMeter and ScopedTimeInterval
Move WaitableEvent
Move ServiceQueue, ThreadLocalValue, SpinLock
Measure CPU utilization in ServiceQueue
Move many Thread related classes