RFR: 8185005: Improve performance of ThreadMXBean.getThreadInfo(long ids[], int maxDepth)

[Daniil Titov]

Please review the change that improves performance of ThreadMXBean MXBean methods returning the 
information for specific threads. The change introduces the thread table that uses ConcurrentHashTable
to store one-to-one the mapping between the thread ids and JavaThread objects and replaces the linear 
search over the thread list in ThreadsList::find_JavaThread_from_java_tid(jlong tid) method with the lookup
in the thread table.

Testing: Mach5 tier1,tier2 and tier3 tests successfully passed.

Webrev: https://cr.openjdk.java.net/~dtitov/8185005/webrev.01/ 
Bug: https://bugs.openjdk.java.net/browse/JDK-8185005 

Thanks!

Best regards,
Daniil