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.

ctl_test.go

@@ -522,7 +522,7 @@ func TestCtl(t *testing.T) {
 	testctl(func(ctl *ctl) {
 		a, b := net.Pipe()
 		go func() {
-			client, err := imapclient.New(a, true)
+			client, err := imapclient.New(mox.Cid(), a, true)
 			tcheck(t, err, "new imapclient")
 			client.Select("inbox")
 			client.Logout()