Do not allow too often orders checking in staggered orders

[bitphage]

On every market update we're getting an event in on_market() callback
inside worker.py which leads to calling
self.workers[worker_name].onMarketUpdate(data) which then calls
Strategy.check_orders() while using staggered oders. This leads to
querying bitshares for each placed order every time market update comes
which may take 3-4 seconds!!!

On active market there are lots of market changes constantly, so the bot
parses new notifications too slow! He just always stays beyond the
actual market. In the active markets like BTS/USD there are lots of
orders happening, but the bot is able to parse only one in 3-4 seconds
(depending on number of orders and lag to the node).

As a quick solution, just limit orders checking rate to once per 5
seconds.

Closes: #169

[MarkoPaasila]

There is no hurry to check the state, so even reducing it to 1440 minutes would be no problem. That's just an extreme, but signifies that this strategy doesn't depend on speed at all.