NEXUS-5418 store sha1/md5 checksums in item attributes

Changed maven2 proxy repositories to store item SHA1 and MD5 hashes
provided by remote repositories as item attributes, not as .sha1
and .md5 items in local storage. This solves common race condition
when multiple threads request the same artifacts at about the same
time and item and its corresponding hashes need to be treated
atomically and locked for entire duration of the operation requesting
the item.

.sha1/.md5 items are not stored/maintained in the local any more but
these files can be generated easily either from attributes or from
item contents if needed.

There is no automatic retry for RemoteStorageException during
.sha1/.md5 remote retrieve. Either expire the artifact or request the
hash asExpired to retry.

It is also not possible to delete .sha1/.md5 items and, generally,
expected/desired behaviour is not clear.

Signed-off-by: Igor Fedorenko <igor@ifedorenko.com>