RFR: Trim the TLAB sizes to avoid wasteful retirement under TLAB races

[Aleksey Shipilev]

http://cr.openjdk.java.net/~shade/shenandoah/tlab-race-trim/webrev.01/

This implements TLAB trimming to avoid wasteful retirement. The rationale is given in the comment
right in the code. It does not seem to make allocation significantly more costly. To make it less
overheady, changed the counter update code to be called on new region issuance, not on TLAB issuance.

Testing: hotspot_gc_shenandoah, allocation benchmarks

Thanks,
-Aleksey