Addressed additional WGLC comments

In particular:
 1) Clarified that HIGHESTMODSEQ/NOMODSEQ only need to be returned upon
    CONDSTORE-enabling.
 2) Clarified when UID is returned in FETCH responses
 3) Clarified that when a STORE/UID STORE UNCHANGEDSINCE is used that
    all affected messsge metadata items need to satisfy the UNCHANGEDSINCE
    criterion, not just any.

Also updated the list of changes since original RFCs.
Changed mod-sequence to be an unsigned 63-bit value

Final -05.