hnix-store-remote: Remove ExceptT, use DList to accumulate log.

[TravisWhitaker]

This PR refactors two bits in the worker protocol implementation:

• A superfluous ExceptT layer is removed. IO is at the base of the stack anyway, so nothing is gained from adding and additional, out-of-band error handling mechanism; callers that care about IO failure will have a set of exception handlers anyway.
• DList is used to accumulate the log. It seems that for long-running interactions with the store there should be a way to stream these log chunks, but this is a partial improvement that isn't opinionated about how the streaming is done.

[sorki]

Thank you, your PR is similar to #52 which uses pipes. We need to get #59 merged and then we can decide on streaming and further improvements.

[Anton-Latukha]

Added a DList benchmarks into: https://github.com/haskell-perf/sequences

[sorki]

Thank you! The ExceptT layer is now used pretty extensively.