Refactor base_uint<>:
* Factor out common code, eliminate redundancies
* Change base_uint::ToString -> to_string(base_uint const&)
* Remove base_uint::GetHex
* Improve STAmount::getText, eliminate unnecessary copying
Use last closed ledger for old pathfinding
Rework handling of modified ledger nodes:
* Track dirty nodes in a set
* Make flushDirty a member function
* Don't write deleted nodes
* Make sure new nodes are shareable
* Put new nodes in the treeNodeCache
Keep SHAMapNodes that fault in handling client requests.