Switch from Java Mail to Apache Commons NET basic SMTP client

This removes the dependency on the encumbered Java Mail client made
by Sun Microsystems, and replaces it with a very simple SMTP library
under the Apache License.

Configuration for the email client is now set in gerrit.config, by
the same properties that "git send-email" would honor.  Unfortunately
we don't support TLS/SSL as a result of this move.

Signed-off-by: Shawn O. Pearce <sop@google.com>