RFR: 8258652: Assert in JvmtiThreadState::cur_stack_depth() can noticeably slow down debugging single stepping

[Chris Plummer]

There is an assert in `JvmtiThreadState::cur_stack_depth()` that tends to slow down single stepping a lot when running the debuggee with a debug jvm. See CR for details. The fix is to allow disabling of this assert using the new EnableJVMTIStackDepthAsserts global, which defaults to true.

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Commit messages:
 - Update copyrights
 - Get rid of asserts that were added to incr() and decre() in previous commit.
 - Add EnableJVMTIStackDepthAsserts global to control the enabling of the JVMTI stack depth asserts.

Changes: https://git.openjdk.java.net/jdk/pull/1835/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=1835&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8258652
  Stats: 13 lines in 2 files changed: 8 ins; 0 del; 5 mod
  Patch: https://git.openjdk.java.net/jdk/pull/1835.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/1835/head:pull/1835

PR: https://git.openjdk.java.net/jdk/pull/1835