* merge 15638: Move CheckTransaction from lib_server to lib_consensus
* merge 15638: Move policy settings to new src/policy/settings unit
* merge 15638: Move rpc utility methods to rpc/util
* merge 15638: Move rpc rawtransaction util functions to rpc/rawtransaction_util.cpp
* merge 15638: Move several units into common libraries
* merge 15638: Move wallet load functions to wallet/load unit
* merge 15638: Document src subdirectories and different libraries
* [build] Add several util units (cleanup)
* build: resolve missing declarations by re-specifying headers