RFR: Make JREInstr to work with instrumentation plugins [v2]

[Alexandre Iline]

> Additional plugin functionality to allow JREInstr to work.
> Tests.

Alexandre Iline has updated the pull request incrementally with one additional commit since the last revision:

  Removed empty test method
  added time logging for the instrumentation
  limited the instrumentation to java.base

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Changes:
  - all: https://git.openjdk.java.net/jcov/pull/17/files
  - new: https://git.openjdk.java.net/jcov/pull/17/files/22cbd776..4d44c31d

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jcov&pr=17&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jcov&pr=17&range=00-01

  Stats: 13 lines in 2 files changed: 5 ins; 6 del; 2 mod
  Patch: https://git.openjdk.java.net/jcov/pull/17.diff
  Fetch: git fetch https://git.openjdk.java.net/jcov pull/17/head:pull/17

PR: https://git.openjdk.java.net/jcov/pull/17