add per-account quota for total message size disk usage

so a single user cannot fill up the disk.
by default, there is (still) no limit. a default can be set in the config file
for all accounts, and a per-account max size can be set that would override any
global setting.

this does not take into account disk usage of the index database. and also not
of any file system overhead.

imapserver/fetch.go

@@ -243,6 +243,7 @@ func (c *conn) cmdxFetch(isUID bool, tag, cmdstr string, p *parser) {
 			err := tx.Update(&mb)
 			xcheckf(err, "updating mailbox counts")
 			cmd.changes = append(cmd.changes, mb.ChangeCounts())
+			// No need to update account total message size.
 		}
 	})
 
@@ -349,6 +350,7 @@ func (cmd *fetchCmd) process(atts []fetchAtt) {
 		m.ModSeq = cmd.xmodseq()
 		err := cmd.tx.Update(m)
 		xcheckf(err, "marking message as seen")
+		// No need to update account total message size.
 
 		cmd.changes = append(cmd.changes, m.ChangeFlags(origFlags))
 	}