Compression and XATTR can crash memcached

A number of places in ep-engine want to examine an xattr document and maybe make changes to it.

• deleteWithMeta needs to 'prune' the xattrs and leave system xattrs
• DCP xattr only/value only needs to modify the item to produce the desired output

Neither of these code paths are snappy aware, they will both only check if the document is xattr, then work on the value as if it has xattrs, however the compressed data can cause the code to make invalid memory accesses and crash.