RFR: Only clear requested gc cause when it is handled

[William Kemper]

Do not unconditionally clear `_requested_gc_cause` on every idle iteration of the control loop as this may erroneously over-write a request to perform a GC cycle. With this change, `_requested_gc_cause` is only cleared when a gc request is handled.

-------------

Commit messages:
 - Only clear requested gc cause when it is handled

Changes: https://git.openjdk.java.net/shenandoah/pull/135/files
 Webrev: https://webrevs.openjdk.java.net/?repo=shenandoah&pr=135&range=00
  Stats: 32 lines in 3 files changed: 25 ins; 6 del; 1 mod
  Patch: https://git.openjdk.java.net/shenandoah/pull/135.diff
  Fetch: git fetch https://git.openjdk.java.net/shenandoah pull/135/head:pull/135

PR: https://git.openjdk.java.net/shenandoah/pull/135