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.

imapclient/prefixconn.go

@@ -0,0 +1,44 @@
+package imapclient
+
+import (
+	"io"
+	"net"
+)
+
+// prefixConn is a net.Conn with a buffer from which the first reads are satisfied.
+// used for STARTTLS where already did a buffered read of initial TLS data.
+type prefixConn struct {
+	prefix []byte
+	net.Conn
+}
+
+func (c *prefixConn) Read(buf []byte) (int, error) {
+	if len(c.prefix) > 0 {
+		n := len(buf)
+		if n > len(c.prefix) {
+			n = len(c.prefix)
+		}
+		copy(buf[:n], c.prefix[:n])
+		c.prefix = c.prefix[n:]
+		if len(c.prefix) == 0 {
+			c.prefix = nil
+		}
+		return n, nil
+	}
+	return c.Conn.Read(buf)
+}
+
+// xprefixConn checks if there are any buffered unconsumed reads. If not, it
+// returns c.conn. Otherwise, it returns a *prefixConn from which the buffered data
+// can be read followed by data from c.conn.
+func (c *Conn) xprefixConn() net.Conn {
+	n := c.br.Buffered()
+	if n == 0 {
+		return c.conn
+	}
+
+	buf := make([]byte, n)
+	_, err := io.ReadFull(c.br, buf)
+	c.xcheckf(err, "get buffered data")
+	return &prefixConn{buf, c.conn}
+}