Making index record size configurable

[AnkurAnand11]

Change log description
Make index record size configurable. Current index record is of fixed length of 4k bytes. It would waste quite some space if real index is small. It is better to be adapted to the real size when reading/writing.

Purpose of the change
Closes #461

What the code does
Created a new method get_record_size in the fields trait of mod.rs with default implementation of 4k bytes. If a different record size value is required, the traits method can be overridden while writing the index event.

How to verify it
All test cases should pass.

[codecov-commenter]

Codecov Report

Merging #459 (511bcda) into master (c0f1813) will decrease coverage by 0.20%.
Report is 1 commits behind head on master.
The diff coverage is 100.00%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #459      +/-   ##
==========================================
- Coverage   72.78%   72.59%   -0.20%     
==========================================
  Files          49       49              
  Lines       12390    12386       -4     
  Branches    12390    12386       -4     
==========================================
- Hits         9018     8991      -27     
- Misses       3372     3395      +23     

Files	Coverage Δ
src/index/mod.rs	96.82% <100.00%> (ø)

... and 12 files with indirect coverage changes

[tkaitchuck]

Where does 111 come from? is there a Java PR that defines it?

[AnkurAnand11]

Where does 111 come from? is there a Java PR that defines it?

We have hardcoded the attribute Id here to 111 since randomly generating it was causing issue while reading.

[tkaitchuck]

Where does 111 come from? is there a Java PR that defines it?

We have hardcoded the attribute Id here to 111 since randomly generating it was causing issue while reading.

These should be defined in the Attributes class in java.

[tkaitchuck]

Humm, this actually brings up an interesting edge case:
Suppose the user creates the reader before the writer. Then we would get a segment does not exist error. In this case we have a couple of options: We could fail and simply not allow creating the reader first.
To make sure the fallback value is correct, we should add a check on reader:350 to make sure the value read was actually the correct size.