Use milliseconds instead of Date for timestamp

[mpapis]

To limit strain on GC we can simplify blockedThreads to include long of milliseconds(System.currentTimeMillis()) instead of Date.

Relevant lines:

• jvm-monitoring-agent/src/com/toptal/jvm/monitoring/Agent.java

  Line 33 in e22e410

  private final WeakHashMap<Thread, Date> blockedThreads = new WeakHashMap<>();

• jvm-monitoring-agent/src/com/toptal/jvm/monitoring/Agent.java

  Line 124 in e22e410

  blockedThreads.putIfAbsent(thread, new Date())

• jvm-monitoring-agent/src/com/toptal/jvm/monitoring/Agent.java

  Line 130 in e22e410

  long now = new Date().getTime();