This PR introduces two new types:

AppendResult - tracks the progress of an individual row append to
completion, either success or error.  Successful appends _may_ have
an associated offset, failed appends will have an associated error.
The AppendResult has a blocking method users can interrogate.

pendingWrite - handles the state management for a set of rows appended
as a group.  There's a 1:many relationship between
pendingWrite:AppenResult(s), so as a pendingWrite completes all
associated AppendResult references should be updated.