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 callingrpc::this_server().stop() from a server procedure (#187)
• Make rpclib compatible with codebases that do not use exceptions
• Addserver::port() to query the port used by the server
• Setreuseaddress option on the server