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
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.
Now SHAMap dumps before it throws an exception.
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.
[FOLD] Simplify and improve performance of STAmount::createHumanCurrency
[FOLD] New strConcat concatenates strings in O(n) time.