3145: Add -readonly flag. Accept multiple -volume arguments.

3145: Add -readonly flag. Accept multiple -volume arguments.

* Leave -volumes (and accept comma-separated dir lists) for backward
  compatibility.

* Detect read-only filesystems when using /proc/mounts to discover
  Keep volumes.

* VolumeManagers know whether volumes are writable, so it's not
  necessary to wait in a serialize queue just to find out a volume
  isn't writable.

* Simplify RRVolumeManager: use atomic.AddUint32() instead of channels
  and goroutines.

* -serialize (like -readonly) now affects only the volumes that come
  after it in the argument list (and automatically discovered
  volumes). This makes it possible to have a mix of serialized and
  non-serialized volumes.

* Fix some test cases that just skipped the test (instead of failing)
  when encountering an error during test setup.

* Move MockVolume code from volume.go to volume_test.go.

* Clean up some over-indented code blocks, see
  https://github.com/golang/go/wiki/CodeReviewComments#indent-error-flow

* Remove some stating-the-obvious/redundant comments and duplicate log
  messages.