MB-15589 indexer: Parallelize index snapshoting for all indexes
MB-15589 indexer: Parallelize index snapshoting for all indexes

For a stable timestamp for a bucket, we create index snapshots
one by one. This causes a significant delay and scalability
bottleneck in index update path proportional to the number of
indexes per bucket. Parallized index snapshot creation operations
for a stable timestamp.

Change-Id: I6b150a3d6de9c5f6769b4102a99cbbce30af01cc

jchris/indexing,jchris/indexing,jchris/indexing,jchris/indexing