Added benchmarks for new registry operations.
Added benchmarks for new registry operations.

These benchmarks cover:

  1. The time taken to make all registered agents unreachable and then
     reachable. This is similar to what happens during a severe network
     partition.

  2. The time taken to GC a significant fraction (50%) of the
     unreachable list from the registry.

Review: https://reviews.apache.org/r/51909/

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