#17 Add 30-seconds interval to optimistic choke

[joaoljr]

This patch adds a 30-seconds interval to the optimistic
choke mechanism, as suggested by the BitTorrent paper
(http://bitconjurer.org/BitTorrent/bittorrentecon.pdf).

Using the m_opt_timestamp to control whether
optimistic choke should be performed is not a very clean
solution and could be improved upon.

This patch is based off version 1.3.4.

[joaoljr]