Reduce byte[] allocation in digest calculation in add entry flow

[mtang01]

FEATURE REQUEST

1. Please describe the feature you are requesting.

This request is an alternative solution to issue #791
The original fix makes 1 byte[] allocation to save more allocations when sending to bookie in v3 protocol.

Alternative solution removes this 1 byte[] allocation, and uses protoBuf ByteString.concat(ByteSring) to piece together protobuf message body. PR to follow.

2. Indicate the importance of this issue to you (blocker, must-have, should-have, nice-to-have).
   Are you currently using any workarounds to address this issue?

No workaround needed. The issue is unnecessary memory allocation.

3. Provide any additional detail on your proposed use case for this feature.