reduce memory footprint

[alaczynski]

When the same module is compiled multiple times then it significantly increases memory usage.
There are few enhancements which could be applied:

1. For each module compilation new ModuleCollector is created to collect instruction coverage of this module instance. This could be skipped when module with same ModuleUri previously compiled is fully covered.
2. If 1st enhancement is not enough then we can introduce configuration parameter which will control how many ModuleCollector instances of the same module (same uri) could be kept in memory. When the threshold exceeds then merging could be performed on the fly.