RFR: Make JREInstr to work with instrumentation plugins

[Alexandre Iline]

Additional plugin functionality to allow JREInstr to work.
Tests.

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Commit messages:
 - Removed unused imports, old version of a test.
 - An instrumentation plugin should also specify what is to be implanted.

Changes: https://git.openjdk.java.net/jcov/pull/17/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jcov&pr=17&range=00
  Stats: 510 lines in 9 files changed: 352 ins; 153 del; 5 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