[vm, isolate] Remove some large safepoint pauses from send-and-exit validation.

 - WeakTable rehashing
 - worklist in the root set

Apply the pattern from object_graph_copy of maintaining the worklist as a heap object to avoid bloating the root set, and using a hash table that can safepoint in the middle of rehashing by not using address-based hash codes.

TEST=IsolateSendExitLatency
Change-Id: I2a632327102b0cc88ccdcffb024d813aa8e355f6
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/372161
Commit-Queue: Ryan Macnak <[email protected]>
Reviewed-by: Alexander Aprelev <[email protected]>

Author: rmacnak-google