Support SO_TIMEOUT configuration in Smtp email sender

Socket SO_TIMEOUT is an important core config in socket communication
allowing to adjust the socket read timeout value.

In Java, when SO_TIMEOUT is set to a non-zero value, the read()
call associated with the socket stream will block until it times out
after the configured timeout by throwing a
java.net.SocketTimeoutException. Otherwise, if SO_TIMEOUT is set 0,
which is the default, it will never timeout.

Sometimes this default may cause issues. In Gerrit, by default, the
'SendEmailExecutor' is a ScheduledThreadPoolExecutor with
corePoolSize=1. Hence when sending emails concurrently, they will be
sent one by one in FIFO order. If sending an email hangs and can't make
progress, other sending tasks will be blocked until the hanging email
is eventually sent or until memory is full (the DelayedWorkQueue used
by ScheduledThreadPoolExecutor is an unbounded queue).

Bug: Issue 40013496
Release-Notes: Support SO_TIMEOUT configuration in Smtp email sender
Change-Id: I4adf69298f34b15c0a56a18cf269abd374597104
Signed-off-by: Dyrone Teng <dyroneteng@gmail.com>