Escape characters in JSON dump in tdb dump

[knutin]

If a field in a TrailDB contains double quotes ("), tdb dump --json does not add the escape parenthesis. This patch handles this particular case, but not other escape characters (like newline).

I tried to keep the change simple and performant. It will only allocate and copy the string if replacement is needed, which most likely will happen very rarely. If no modification is needed, no allocation or copy is done.

I'm a bit unsure about the convention around freeing memory so I'm happy to take another pass if there's a better way.

As a side note, there's quite a few different ways we could speed up the replacement and I'd be happy to do another pass to improve performance.

[tuulos]

This is a tricky problem. Currently tdbcli makes no attempt in producing very "valid" CSV or JSON.

Especially producing valid JSON is hard, since the RFC requires that JSON text is encoded in UTF-8 (or 32). Converting arbitrary binary data to valid unicode is tedious in C.

One option that I have in mind is to provide an option to urlencode everything. This would guarantee (more) valid JSON and CSV (urlencode in ascii is valid UTF-8) and urlencoding is somewhat human readable.