To help diagnose e-mail-related issues, EmailMessage.send() now logs

the time taken for the e-mails to go out. This logs the recipient

message and the subject, to help identify the message.
This is all logged as debug logs, so in production this shouldn't create

too much additional noise unless debug messages are needed. However, it

will make it clear via warning and critical logs if there are delays

with sending.

Unit tests pass.
Tested this with some e-mail sending code in Review Board, verifying the

log messages.