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/prefixconn.go

@@ -1,6 +1,8 @@
 package imapserver
 
 import (
+	"bufio"
+	"io"
 	"net"
 )
 
@@ -26,3 +28,18 @@ func (c *prefixConn) Read(buf []byte) (int, error) {
 	}
 	return c.Conn.Read(buf)
 }
+
+// xprefixConn returns either the original net.Conn passed as parameter, or returns
+// a *prefixConn returning the buffered data available in br followed data from the
+// net.Conn passed in.
+func xprefixConn(c net.Conn, br *bufio.Reader) net.Conn {
+	n := br.Buffered()
+	if n == 0 {
+		return c
+	}
+
+	buf := make([]byte, n)
+	_, err := io.ReadFull(c, buf)
+	xcheckf(err, "get buffered data")
+	return &prefixConn{buf, c}
+}