Add initial transactWriteItem implementation [WIP]

[tume]

Started implementing transactWriteItem by mimicking batchWrite function with few modifications. At the moment it is a very crude start. I mainly wanted to get this out to see if this is even the right direction to pursue and maybe also get some advice on how to continue this if this is a valid way to go.

Does not handle ConditionCheck or rollback at all and didn't add any tests either :). Validation probably needs also improvement.

related to #98

[Streadz]

hi @tume! Just wanted to check in on this PR you made. Did you plan on still getting feedback from maintainers about your progress so far?

[mpangu]

@tume Are you planning to merge this change ?

[tume]

I don't have write access to this repository and not actively using anymore this library... probably still needs some work before merging and requires some advice from the author.

[MrCheater]

@mhart Please, check the pull request

[mhart]

Won't merge this without tests and some more fleshing out, but it looks like a good start!

[bwitt]

I'm interested in seeing this in dynalite!

[mtimbs]

Any update on this? What do we need to do to get this merged in?

[joffarex]

After all this time... Is there any news? @MrCheater @mhart could someone who knows the library take on this and work on implementation?

[ryanblock]

Given that the author of this PR mentioned moving on from usage of this project, and @jpeterschmidt has an even deeper one baking at #143, I'm going to say we should let PR supersede this one. Please let me know if you disagree for whatever reason!