Fixes

• Fix compile error on gcc 4.9
• Fix warnings in clang 7
• Fix self-assignment
• Fix early destruction of server sessions
• Fix crashes in multithreaded environment (#175)

Additions

• Support calling rpc::this_server().stop() from a server procedure (#187)
• Make rpclib compatible with codebases that do not use exceptions
• Add server::port() to query the port used by the server
• Set reuseaddress option on the server