RFR: Rename flag to ShenandoahGuaranteedGCInterval

[Aleksey Shipilev]

This is for two reasons:
 a) It really is the interval, not period: measures time since last gc *end*, not from the *start*,
like the period would do;
 b) Has some symmetry with other VM options, like GuaranteedSafepointInterval

Webrev:
 http://cr.openjdk.java.net/~shade/shenandoah/periodic-gc-rename/webrev.01/

Testing: hotspot_gc_shenandoah

Thanks,
-Aleksey