Parse packages in parallel

Introduced ParallelPackageParser. Parsing requests are processed by a
thread-pool (currently 4 threads). At any time, at most 10 results are kept
in RAM. This is enforced by the blocking queue.

scanDir has become a two-stage process:
1) Collects files for parsing and submit them to the parallel parser
2) Sequentially take elements from the parsing queue and process them by
   calling scanPackageLI (as before)

Test: manual - device boots, all packages are parsed
Test: ParallelPackageParserTest passes
Bug: 30792387
Change-Id: I07a64da4d83e355b2b1f1ab350e6d8087dfd2feb