implement gateway batch RFC

[tbg]

I'm going to start a series of PRs towards implementing #1998. First step will be changing (*DistSender).Send() to auto-wrap what it receives in a single-element Batch and unroll it at (*Store).ExecuteCmd()`.

[tbg]

Have the above working with tests passing, but still some code cleanup to do before I post it (a lot of assumptions need rewiring for batch: truncation/combining and concepts of read only/etc).