Levelize SHAMap:

The SHAMap class is refactored into a separate module where each translation
unit compiles separate without errors. Dependencies on higher level business
logic are removed. SHAMap now depends only on basics, crypto, nodestore,
and protocol:

* Inject NodeStore::Database& to SHAMap
* Move sync filter instances to app/ledger/
* Move shamap to its own module
* Move FullBelowCache to shamap/
* Move private code to shamap/impl/
* Refactor SHAMap treatment of missing node handler
* Inject and use Journal for logging in SHAMap