Pathfinding dispatch improvements
* Simplify decision whether to update a path
* Prevent pathfinding threads from blocking each other
Set version to 0.24.0-rc2
Set version to 0.24.0
Update RPM spec file to version 0.24.0
Cleanup nodestore backend classes:
* Add README.md
* Add missing std::move calls
* Refactor visitAll in backend
Rename Feature to Amendment:
* Added a README.md describing an Amendment
Simplify and improve performance of STAmount::createHumanCurrency
Add documentation to SHAMap
Clean up of TransactionSign and friends.
New strConcat concatenates strings in O(n) time.

* Accepts std::string and char const*.
* Also accepts numbers, bools and chars.
* New ripple::toString function augments std::to_string to handle
  bools and chars.
Reduce number of async fetches
Improved performance monitoring of NodeStore backends:
* Add multi-sample interface to LoadMonitor
* Instrument fetch operations and completions for reporting in server_info
Use Json::StaticString to improve performance:
* Move JSON-RPC fields to jsonrpc_fields.h
Implement C++14 std::make_reverse_iterator
Refactor base_uint<> and STAmount::getText:

base_uint<>:
* Remove unnecessary typedefs and derived classes
* Factor out common code and eliminate redundancies
* Add tag support to improve type safety
* base_uint::ToString() -> to_string(base_uint const&)
* base_uint::GetHex() -> to_string(base_uint const&)

STAmount::getText:
* Don't overallocate
* Eliminate unnecessary copying
Use last closed ledger for old pathfinding
Rework handling of modified ledger nodes:
* Track dirty nodes in a set
* Make flushDirty a member function
* Don't write deleted nodes
* Make sure new nodes are shareable
* Put new nodes in the treeNodeCache
Keep SHAMapNodes that fault in handling client requests.
Refactor RippleCalc.cpp:

* Add comments
* Restrict code to 80 colums
* Remove boost::format
* Remove BOOST_FOREACH
* Make members private
* Add ripple_unordered_set to UnorderedContainers.h
* Replace boost::unordered_set with ripple::unordered_set
Add experimental offer cross templates:
  * Also includes experimental SConstruct changes