Better handling/treatment of expensive search queries

[krauselukas]

Issue Description

At the moment we allow very expensive search queries on the OBS API on endpoints like search/request. Those can take hours in order to be processed and can become very memory hungry.

Expected Result

Search queries that take hours or consume to much memory shouldn't be possible (limited) or better handled.
Possible solutions could be only allowing more specific search request by introducing scopes or simply starting with timeouts.

How to Reproduce

An example would be the following search query for the mentioned endpoint: params={"match"=>"action/target/@project='openSUSE:Factory' or action/source/@project='openSUSE:Factory'"}