RFR: 8352042: [leyden] Parallel precompilation

[Aleksey Shipilev]

For simplicity reasons, our current precompilation code uses effectively single-threaded blocking compiles. Even for simple cases, this makes assembly phase remarkably long. This does not scale well. We need to do precompilations in parallel. 

Unfortunately, I haven't found a good way to check that all async compilations are completed, so I introduced a new one.

Additional testing:
 - [x] Ad-hoc precompilation tests
 - [x] Linux x86_64 server fastdebug, `runtime/cds`

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Commit messages:
 - Call CompileBroker not CompilerTask
 - Fix
 - Fix

Changes: https://git.openjdk.org/leyden/pull/49/files
  Webrev: https://webrevs.openjdk.org/?repo=leyden&pr=49&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8352042
  Stats: 123 lines in 8 files changed: 65 ins; 44 del; 14 mod
  Patch: https://git.openjdk.org/leyden/pull/49.diff
  Fetch: git fetch https://git.openjdk.org/leyden.git pull/49/head:pull/49

PR: https://git.openjdk.org/leyden/pull/49