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SHAMap performance improvements (RIPD-434)

This reworks the way SHAMaps are stored, flushed, backed, and
traversed. Rather than storing the linkages in the SHAMap itself,
that information is now stored in the nodes. This makes
snapshotting much cheaper and also allows traverse work done on
behalf of one SHAMap to be used by other SHAMaps that share inner
nodes with that SHAMap.

When a SHAMap is modified, nodes are modified all the way up to the
root. This means that the modified nodes in a SHAMap can easily be
traversed for flushing. So they don't need to be separately tracked.

Summary
* Remove mTNByID
* Remove mDirtyNodes
* Much faster traverses
* Much Faster snapshots
* New algorithm for flushing
* New vistNodes/visitLeaves
* Avoid I/O if a map is unbacked
Merge pull request #145 from graydon/ripple-labs-shamap-refactor

SHAMap performance improvements (RIPD-434)

Reviewed-by: jedmccaleb
WIP for dumping serialized ledgers and txs to json file.
WIP sketch of --load_transactions mode
Remove Ledger::copyAccountStateFromLedger
Add accessors to Ledger
Factor dumpTransactions, add dumpLedger.
Ledger 113 inflates, w/fee=10; return to charging fee.
Re-enable checking account, tx hashes on replay.
Use cout for logging on replay so ledgers aren't truncated.
Ledger 4251 closed at 00:19:21 w/sLCF_NoConsensusTime, need to propagate.
Ledger 19205 makes a trustline with asymmetric metadata.
Ledger 78742 applies txs in non-standard order in later phases.
Revert "remove lsfPasswordSpent", as ledger 80960 uses that flag.

This reverts commit 054c0a848af89f1e76c64f0aac424eadf7bca007.
Ledger 81011 uses lsfDisableMaster.
Ledger 87668 has a very large and malformed path that is only judged "dry".
Disable node caches in DatabaseImp