PeerFinder and Peers fixes:
- Fix bootcache incorrect failure on socket cancel
- Fix peer connect race on stop
- Simplify some loops
- Require boost 1.55 via static_assert
Remove unused classes
Fix clang warnings
Give pathfinding updates priority over client and RPC requests
WebSocket dispatch changes.
Don't let one connection hold a thread for too long without
having to wait for another turn
Standardize on module db path & pass it to Validators and PeerFinder
Improve I/O when fetching ledgers
* negative cache for node store
* async fetch, thread pool for node store
* read barrier logic for node store
* SHAMap getMissingNodesNB (non-blocking)
* non-blocking getMissingNodes traverse
* tune caches
Release the InboundLedger lock during getMissingNode
Don't check the node store for proposed txn nodes