[docdb] Investigate using PB arena allocations for local bootstrap

[bmatican]

Jira Link: DB-1570
As part of the same investigation that lead to #2124, I also noticed some 20% CPU time spent in destroying PB items that were created as part of reading the log entries (and of course some corresponding amount of time in allocations).

This might be an interesting case to try to put in PB arena allocations. While it would not help with the underlying strings being created, it should at least help with the chain of PBs (LogWriteBatchPB, LogEntryPB, WriteRequestPB, WriteBatchPB, KeyValuePB ...)

cc @rahuldesirazu @spolitov