Split Section to its own header and add convenience accessors
Move some constants to core/SystemParameters.h
Add fee voting configuration and docs (RIPD-564)
SHAMap performance improvements [RIPD-434]
 * Remove mTNByID
 * Remove mDirtyNodes
 * Much faster traverses
 * Faster snapshots
 * New algorithm for flushing
 * New vistNodes/visitLeaves
 * Avoid I/O if a map is unbacked