Remove unused SHAMap::getItem
Add missing beast includes
Move nodestore to its own module
Tidy up overlay declarations
Use strand::running_in_this_thread instead of bool
Move overlay up one directory
Remove unused protocol message types:
* mtACCOUNT
* mtCONTACT
* mtERROR
* mtGET_ACCOUNT
* mtGET_CONTACTS
* mtGET_VALIDATIONS
* mtSEARCH_TRANSACTION
* mtUNUSED_FIELD
[WIP] Update README.md
Add abstract_protocol_handler, message_stream