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/compress_test.go

@@ -0,0 +1,39 @@
+package imapserver
+
+import (
+	"crypto/tls"
+	"testing"
+)
+
+func TestCompress(t *testing.T) {
+	tc := start(t)
+	defer tc.close()
+
+	tc.client.Login("mjl@mox.example", password0)
+
+	tc.transactf("bad", "compress")
+	tc.transactf("bad", "compress bogus ")
+	tc.transactf("no", "compress bogus")
+
+	tc.client.CompressDeflate()
+	tc.transactf("no", "compress deflate") // Cannot have multiple.
+	tc.xcode("COMPRESSIONACTIVE")
+
+	tc.client.Select("inbox")
+	tc.transactf("ok", "append inbox (\\seen) {%d+}\r\n%s", len(exampleMsg), exampleMsg)
+	tc.transactf("ok", "noop")
+	tc.transactf("ok", "fetch 1 body.peek[1]")
+}
+
+func TestCompressStartTLS(t *testing.T) {
+	tc := start(t)
+	defer tc.close()
+
+	tc.client.Starttls(&tls.Config{InsecureSkipVerify: true})
+	tc.client.Login("mjl@mox.example", password0)
+	tc.client.CompressDeflate()
+	tc.client.Select("inbox")
+	tc.transactf("ok", "append inbox (\\seen) {%d+}\r\n%s", len(exampleMsg), exampleMsg)
+	tc.transactf("ok", "noop")
+	tc.transactf("ok", "fetch 1 body.peek[1]")
+}