Refactor libsyndicate/ms-client.cpp

[jcnelson]

See if we can split out the Volume control-plane code from the File/xattr "metadata plane" code. They should probably go into separate modules.

[jcnelson]

Other refactoring objectives:

• use the mddownload* API whenever possible. Reduce use of curl handle multiplexing, and consolidate low-level HTTP code
• eliminate busy-waiting for volume metadata refreshes
• separate module for URL generation
• separate module for xattrs (to include xattr-based query API)
• separate module for path resolution
• separate module for cert processing
• separate module for vacuuming
• separate module for file CRUD
• separate module for core data structures and getters/setters

[jcnelson]

The only things left as of 69b5c9db3bddb4b8bfe4261da04e12cab5592541 are separating the code into distinct modules.

[jcnelson]

Also, fetch gateway certificates in parallel

[jcnelson]

Fixed on f5750c794b0cffaa23d346688a9abd5a56a3e3a2