[ORC] Move SimpleRemoteEPCServer::Dispatcher into OrcShared.

Renames SimpleRemoteEPCServer::Dispatcher to SimpleRemoteEPCDispatcher and
moves it into OrcShared. SimpleRemoteEPCServer::ThreadDispatcher is similarly
moved and renamed to DynamicThreadPoolSimpleRemoteEPCDispatcher.

This will allow these classes to be reused by SimpleRemoteEPC on the controller
side of the connection.