Excessive time for "body exists" HEAD request on sparse volume.

[DocSavage]

HEAD request on sparsevol endpoint (labelvol type) is very slow, even compared to what should be faster GET sparsevol requests. See janelia-flyem/libdvid-cpp#14 Check for errors. Also, on finding any RLE, the sequential read query should be aborted. This will require changes at the storage API level to allow an abort signal for channel-based sequential scans.