CompilationUnitCache: gracefully cleanup the name environment

[BoykoAlex]

Too many exceptions coming from the CU cache due to disposed name environment in the middle of CU parsing or working on the CU with reconcilers or symbol scanners.

Name environment on the CU cache is cleared when an event affecting this environment arrives, i.e. classpath change or class file changes.

Let the work in progress on the CU finish and then update the name environment:

• reduce the # of weird exceptions
• let reconciling, symbol scanning, syntax highlighting finish normally and provide results in case the operation won't be reattempted

[BoykoAlex]

Fixed with 2543bdf