[Cosmetic]

Create Server instance on stack instead of the heap. This makes the object's lifetime clear: It exists until end of main().
Delete ::socketFile which was just a copy of server.options().socketFile.
[Cosmetic]
[Cosmetic] minor changes to Server::restoreFileIds()

One less variable (clear).
Always close the file - previously not done if wrong database version read. (I think the best would be for a destructor to do the file close!)
Handle SIGTERM - handle the same as SIGINT.
REAME.org: add link to vim-rtags.
.

Pass Server::Options to Server constructor rather than Server::init().
.

Remove extraneous "-Wall" compiler option. Do it just once with:
  set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wall")
.

Delete something related to "gr" executable which is no longer built. (One of its source files, GRParser.cpp, was deleted in a48069ed01...)
rdm is now a daemon.
.

Use new fileContents() function instead of Path::readAll() for reading rc config files: Led to simplifying code populating rcArgs (previously argCopy) list.