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.
