steam: do not handle timestamps on polled messages

Originally, when timestamp handling was implemented, it was done with
the intent of knowing exactly when that message was created. However,
if anything, the handling timestamps on polled messages has just led to
problems. Timestamps are not relevant when holding a conversation, even
when latency gets in the way. Additionally, no other BitlBee protocol
plugin handles timestamps on polled messages, so this plugin shall do
the same.

This does *not* remove the handling of timestamps on the unread messages
retrieved upon login, only polled messages.
steam: immediately store the last message timestamp

Currently, the last message timestamp is stored locally, and only stored
in the set interface upon the account logging off. This becomes an issue
when BitlBee is unexpectedly killed, and the account was not logged off.

The solution is to directly work with the set interface for storing the
last message timestamp. It is possible this will not completely address
the issue, and a force save will need to be done when the set value is
updated. However, this is not implemented in this patch, and will only
be done as as last resort due to the overhead.
steam-api: increase the poll timeout to 30 seconds

Originally, the poll timeout was dropped to 15 seconds to account for
latency and relogons. However, with bitlbee-3.2.1 and the introduction
of HTTP-1.1 support, latency should no longer be an issue.

For now, use a timeout of 30 seconds, however, it is possible the past
issues will surface again.