Implement support for x-amz-meta (bb #884)

avoid calculating md5 in some situations
copy meta too
be consistent in quoting ETags

use etag based on SX file revision where possible
avoids querying each file in volume listing, and provides a sane etag
for files uploaded from SX.
fix etag in list parts

update tests
s3cmd info support

return dummy bucket policy
fix 404 reply on nonexistent object
fix ENOENT on delete
workaround for fake dir creation

using .sxnewdir
reply with entity too large on quota exceeded (bb #904)

don't return a body on HTTP 416
don't try to send headers again if we already sent it
for example if an error occurs in multipart complete
we can only send an error xml in the body because the header
was already sent.
Also send xml header only once.
fix HEAD reply
simplify tests

store test files in memory instead of the FS, dropping a lot of code that was
only used for the tests
fix default volume size arg
wait for delete to finish on SX side
fast remote to remote copy

mput_delete concurrency

don't retrieve both filemeta and file just to determine etag and filesize

store filesize as meta too, and retrieve it with just the filemeta
basic testsuite
some tests
fix reply namespace for object copy
store Content-Type as meta
test for dev version of LibreS3 with dev version of SX