Append index for CSV

[friendlymatthew]

Fixes #39

The problem

To append a new index file, ReadIndexFile is called. ReadIndexFile proceeds to build the IndexFile from the data generated by NewIndexFile. To append new data that exists in ReadIndexFile but not in NewIndexFile, we call Synchronize. The problem is for CSV files, we need to pass in the headers from ReadIndexFile.

The solution

1. Extract headers from data passed in from ReadIndexFile.

We could have spliced the data from 0 -> f.EndByteOffsets[0]. But since we want to support a schemaless design, if the headers change, we want to catch for this. So we need to use csvReader to parse the header.

2. Pass headers to Synchronize

Synchronize gets called in two different places:

 1. Index file creation, here we can strip the head line and treat it as our header
 2. Read index file, which needs explicit header data passed

Next steps

• write more tests, the existing append index file test passes
• clean up, remove debug comments