Buffer IMAP client writes

async-imap does not do its own buffering, but calls flush() after
sending each command. Using BufWriter reduces the number of write()
system calls used to send a single command.

Note that BufWriter is set up on top of TLS streams, because
we can't guarantee that TLS libraries flush the stream before
waiting for response.

CHANGELOG.md

@@ -9,6 +9,7 @@
 - cargo: bump quick-xml from 0.23.0 to 0.26.0 #3722
 - Add fuzzing tests #3853
 - Add mappings for some file types to Viewtype / MIME type #3881
+- Buffer IMAP client writes #3888
 
 ### API-Changes
 - jsonrpc: add python API for webxdc updates #3872