Support SMTP CHUNKING extension

Sending large messages over SMTP is often slow due to the need to line-wrap content.

The CHUNKING extension (RFC 3030) allows for a BDAT command to replace the DATA command. This allows for messages to be sent as continual chunks of data rather than line-wrapped.

It is supported by Gmail and other providers.

Implementation of this will need to be aware of the PIPELINE extension (see #2338) and 8BITMIME extension.

We will need to pick an appropriate chunk size. This might involve investigation of what other providers use (the RFC uses 100000 but it's unclear whether this figure was arbitrarily chosen). It seems over-the-top to allow it to be configurable.

Obviously if the extension instead present we will need to use the normal data command.

Tests should cover use and lack of the extension, as well interaction with PIPELINING

Start by tracing the SMTP implementation's DATA handling and reviewing the existing PIPELINE support referenced by issue #2338. Read RFC 3030 and determine how CHUNKING interacts with PIPELINING and 8BITMIME; done means using BDAT when advertised, falling back to DATA otherwise, with tests for both paths and their interaction.