Implement maximum search queue length

[clamprecht]

To avoid situations where an index gets overloaded with searches (sometimes a slow search can cause this), I implemented an option called "max_search_queue", which sets a limit of the number of threads waiting to acquire the search semaphore in TrafficLimitingSearcher. If this limit is reached, it just rejects the search and throws InterruptedException instead of melting down. I've been running this in production and it seems to work well.

If this sounds good, I'll clean this up into a nice pull request, but here's the main code change:

clamprecht@1e766e2

The option name I chose in the indexengine_config is "max_search_queue".

[santip]

It sounds great. Another alternative could be to limit the amount of time a Thread should be waiting. Either way is better than the current state. Please clean it up and we'll merge it.

Thanks

[searchify]

Pull request submitted in issue #37