[Java] Add getInitReservation() to BufferAllocator interface similar to getLimit(), getHeadRoom() APIs

[asfimport]

For capturing additional information for debugging/profiling purposes, it will be useful to expose the init reservation for buffer allocator. 

I would encourage someone new to the community to do this.

Reporter: Siddharth Teotia / @siddharthteotia

_{Note: This issue was originally created as ARROW-2001. Please see the migration documentation for further details.}

[asfimport]

Arun Shanmugam Kumar:
@siddharthteotia, I can pick this up if nobody is working on it.

Thanks,
Arun

[asfimport]

Siddharth Teotia / @siddharthteotia:
[~arunsk], please feel free to work on this. Let me know if you have any questions.