Remove rocksdb in preparation for subtree add
Squashed 'src/rocksdb/' content from commit 224932d

git-subtree-dir: src/rocksdb
git-subtree-split: 224932d4d0b561712107d747c662df181c39644d
Merge commit 'f86d9fd626df1cee55bce4c577d06bb064dc827b' as 'src/rocksdb'
Update rocksdb unity file (RIPD-352)
Fix macro setting for rocksdb unity file
Detect node store batch write failures (RIPD-270):
* Raise open file limit from the soft max to the hard max.
Remove obsolete nickname support
Cleanups and performance optimizations (RIPD-450):
* Remove AccountItems and AccountItem
* Restructure RippleLineCache to not require shared_ptr
* Avoid expensive copies of base_uint<160> in RippleState
Update Doxyfile (RIPD-175):
* Clean up the include path to reflect updated code structure.
* Update PROJECT_BRIEF to more accurate title.
* Fix location of logo graphic.
* Hide all undocumented classes.
Refactor beast::SemanticVersion (RIPD-199)
Bump ledger_wait timeout for Travis
Add the git tag to the compile (RIPD-238)
Fix Pathfinder::getPathsOut to use Issue
Remove unused code:
* StringConcat was only being referenced by unit test.
* `runall.sh` is no longer needed. Use `npm test` instead.
Refactor SField (RIPD-431)
* Restrict access to SField constructors.
* Make all SField access const.
* Hide and simplify databases used to hold SField constants.
* Separate the two concerns of representing a field,
  and maintaining a database of fields.