Increase the size of the tree cache:
This change will not significantly increase memory consumption
because most entries are pinned anyway.
Make more RocksDB tunables
Add support for universal compaction
Refactor LedgerEntrySet:
* Split adjustOwnerCount to increment and decrement paths.
* Move pathfinding-specific functions out of LedgerEntrySet
* Convert members to free functions
Improve regularity of STObject and STArray (RIPD-448, RIPD-544):
* reduce duplicated code using templates
* replace BOOST_FOREACH with C++11 for loops
* remove most direct calls to new
* limit line length to 80 characters
* clearly identify virtual and overridden methods
* split STObject and STArray into their own files
* name files after the class they contain