Cleanup confusion of ledger base versus ledger header

The "ledger header" is the chunk of data that hashes to the
ledger's hash. It contains the sequence number, parent hash,
hash of the previous ledger, hash of the root node of the
state tree, and so on.

The term "ledger base" refers to a particular type of query
and response used in the ledger fetch process that includes
the ledger header but may also contain other information
such as the root node of the state tree.
Remove boost::hash_value() overloads.
This addresses https://ripplelabs.atlassian.net/browse/RIPD-102
Fix base log partition severity
Add preliminary Structured Overlay docs
Remove rockdb subtree
Squashed 'src/rocksdb/' content from commit 78b8a7d

git-subtree-dir: src/rocksdb
git-subtree-split: 78b8a7d908d7e67cc6664398e39560b76c965773
Merge commit '7bfb4a9ba5a2822f7d9ef7122b1388aea4be9404' as 'src/rocksdb'
Update rocksdb unity file
Remove recursions from computeLiquidity
Rename RippleAsset to Issue and RippleBook to Book:
* Split STAmount out of SerializedTypes.h
* New concept of "Issue consistency": when either both or neither of its
  currency and account are XRP.
* Stop checking for consistency of Issue in its constructor.
* Clarification of mIsNative logic in STAmount.
* Usual cleanups.
Pathfinding Documentation
SHAMap Documentation
Patch Boost to suppress warnings on clang
Add documentation for ledger entries (RIPD-361)
Impose a local limit on path lengths (RIPD-350)
Find 'sabfd' paths (RIPD-335)
This permits USD/GW1 to be bridged to USD/GW2 by BTC/GW3 or
USD/GW1 to be bridged to BTC/GW2 by CNY/GW3.
Fix account_tx resume (RIPD-314)
Various tidying:
* Fix unused variable warnings.
* Clean unused items from TER.
* Improvement to LES use of shared_ptr.