V2 ReadEntryProcessor should handle asynchronous fence like what V3 does

[sijie]

FEATURE REQUEST

1. Please describe the feature you are requesting.

Address TODO item in ReadEntryProcessor:

            // TODO:
            // currently we don't have readCallback to run in separated read
            // threads. after BOOKKEEPER-429 is complete, we could improve
            // following code to make it not wait here
            //
            // For now, since we only try to wait after read entry. so writing
            // to journal and read entry are executed in different thread
            // it would be fine.

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?

nice-to-have

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

N/A

[ivankelly]

In general ReadEntryProcessor#processPacket needs a bit of a refactor to use java 8 features. This change should be pretty straightforward though.