DWDS/Dialing Concordance Code

- string_socket.cpp: 1 send() per message
- set TCP_NODELAY during create_socket() to disable buffering
- BranchServer: don't send get_hit_strings() if there's nothing to retrieve
+ TODO: stop subcorpora from complaining about this (Error Cannot send string: Broken pipe; errno=32)
- joint work with Adrien Barbaresi