Initialization of some of the fields in JavaPlugin was not thread safe.

This change tries to add thread safe init without holding extra locks in
critical paths to prevent possible deadlocks while loading 3rd party
code.

This may result in possible double creation of some objects but should
avoid errors on multiple threads initializing same JavaPlugin fields in
parallel.

Fixes https://github.com/eclipse-jdt/eclipse.jdt.core/issues/2276