Checksum

[danielballan]

HTTP supports a Want-Digest request header and a Digest response header. The tiled server should support these, and the tiled Python client should support optional digest verification after download.

[danielballan]

Note: as an implementation detail, we already compute the md5 digest for most ETag headers, though this should not be generally relied on. I wonder if there is a clean way to avoid computing the md5 twice though, in those instances where ETag and Digest would happen to be the same.