Remove usage of parallelStream in the client

[poojabhat0]

Change all usages of parallelStream to stream in the client as it uses the common ForkJoinPool and probably hurts more than it helps.
Reference: https://stackoverflow.com/questions/20375176/should-i-always-use-a-parallel-stream-when-possible/23370799#23370799

[achals]

Ideally we'd accept an executor service in the builder/constructor and callers can pass in one that they need, in case we needed parallelism?