RFR: 8259242: Remove ProtectionDomainSet_lock [v4]

[Coleen Phillimore]

> This change makes the DictionaryEntry pd_set reads lock free, because when I set a low SafepointTimeout on the findDeadlock test, these threads were waiting on the ProtectionDomainSet_lock.  This lock is a singleton that's taken for every dictionary entry.
> 
> I don't know if this change will make the test never timeout again, because removing this lock caused us to hit a low SafepointTimeout on another _no_safepoint_check lock that is not so easy to eradictate.
> 
> Tested with tiers 1-3.  There's a test that exercises this code in runtime/Dictionary/ProtectionDomainCacheTest.java.

Coleen Phillimore has updated the pull request incrementally with one additional commit since the last revision:

  Add overall comment and assert in count function.

-------------

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/3362/files
  - new: https://git.openjdk.java.net/jdk/pull/3362/files/06624ed1..153eccd7

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=3362&range=03
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=3362&range=02-03

  Stats: 12 lines in 1 file changed: 11 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3362.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3362/head:pull/3362

PR: https://git.openjdk.java.net/jdk/pull/3362