Sync 'isDefined' in presentation compiler cache.

This fixes one race condition when hyperlinking between
projects. Project 'client' depends on project 'base'.
The sequence of events was:

- dependent project 'base' starts building
- the indexer for `client` kicks in and starts the PC. The PC is not
  yet initialized. PC starts without `base` classfiles on its class path
- `base` finished building and triggers a reset of the PC for `client`,
  that normally restarts with `base` classfiles on the class path
- reset is ignored (PC is still starting, and `isDefined` returns false)
- hyperlinking fails, because the target is not available on the class path
Merge pull request #491 from huitseeker/fix/race-condition-cached-juno

[Backport] Sync 'isDefined' in presentation compiler cache.