merge: redistribute Context and Mailbox

 * fix mxapi comments
 * move Context.mx_ops
 * move Context.ctx->data
 * move Context.{hdrs,hdrmax}
 * move Context.{size,changed,readonly,quiet,closing}
 * free_data
 * notmuch: move mailbox data
 * notmuch: let mxapi do cleanup
 * notmuch: use mailbox, not context
 * mbox: use Mailbox, not Context
 * maildir: use Mailbox, not Context
 * nntp: use Mailbox, not Context
 * pop: use Mailbox, not Context
 * compress: use Mailbox, not Context
 * imap: use Mailbox, not Context
 * add mbox private Mailbox data
 * move Context.rights
 * eliminate Context from backends
 * Mailbox doesn't begin with 'b'
 * move Context.compress
 * tidy compress
 * move Context.{id,subj,label}_hash
 * move Context.{v2r,vcount}
 * move Context.mtime
 * tidy context usage
 * imap: rename mailbox out of the way