implement IMAP extension COMPRESS=DEFLATE, rfc 4978

to compress the entire IMAP connection. tested with thunderbird, meli, k9, ios
mail. the initial implementation had interoperability issues with some of these
clients: if they write the deflate stream and flush in "partial mode", the go
stdlib flate reader does not return any data (until there is an explicit
zero-length "sync flush" block, or until the history/sliding window is full),
blocking progress, resulting in clients closing the seemingly stuck connection
after considering the connection timed out. this includes a coy of the flate
package with a new reader that returns partially flushed blocks earlier.

this also adds imap trace logging to imapclient.Conn, which was useful for
debugging.

imapserver/fuzz_test.go

@@ -120,7 +120,7 @@ func FuzzServer(f *testing.F) {
 
 				err := clientConn.SetDeadline(time.Now().Add(time.Second))
 				flog(err, "set client deadline")
-				client, _ := imapclient.New(clientConn, true)
+				client, _ := imapclient.New(mox.Cid(), clientConn, true)
 
 				for _, cmd := range cmds {
 					client.Commandf("", "%s", cmd)