Pathfinding dispatch improvements
* Simplify decision whether to update a path
* Prevent pathfinding threads from blocking each other
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.