Check how parallel instrumentation sessions can be handled

External build will start/close single instrumentation session in which modules will be compiled in parallel (see CLOV-1267).

However, a parallel build in ant/maven might involve starting multiple instrumentation sessions at once (just think about <parallel> + <javac> in Ant for instance).

Check how we could deal with this. Possible problems:

• clover.db file locking (appending instrumentation session)

Areas for prototyping:

• clover db shared in memory
• clover db structure change - adding top-level index for sessions/files/classes etc, possibility to interleave data from several instrumentation sessions
• etc ...