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
Squashed 'src/snappy/snappy/' content from commit 1ff9be9

git-subtree-dir: src/snappy/snappy
git-subtree-split: 1ff9be9b8fafc8528ca9e055646f5932aa5db9c4
Merge commit 'd5ebc515e4b382741ec2f6823610e26e5d811787' as 'src/snappy/snappy'
Add .gitattributes for src/snappy
Add snappy compression integration
[FOLD] WTF?