Implementation of sharding for procstats atoms

Oftentimes, the ProcStats protobuf is larger than StatsEvent can handle.
With this CL, we detect that state before it happens, and attempt to
shard the ProcStats data over multiple protobufs, each with its own
StatsEvent.

Test: see b/158294266
Bug: 158294266
Change-Id: I1787d0ddeb6825a55adb8e0e1ec55c89af6990c5