abci: ADR-008 Merge abci-rs into tendermint-rs

docs/architecture/adr-008-abci-rs-merge.md

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+# ADR 008: Merge `abci-rs` into `tendermint-rs`
+
+## Changelog
+* 2020-08-11: First draft
+
+## Context
+
+As mentioned #388 one of the high level goals for the `tendermint-rs` repo is to merge an already existing crate to build ABCI servers with. There are three existing crates for this purpose:
+
+1. [`rust-abci`](https://github.com/tendermint/rust-abci)
+1. [`abci-rs`](https://github.com/devashishdxt/abci-rs)
+1. [`abci2`](https://github.com/nomic-io/abci2)
+
+### History
+
+There was a [proposal](https://github.com/tendermint/rust-abci/issues/61) for rearchitecting `rust-abci` where a lot
+of different aspects about `rust-abci` were discussed (for example, current `Application` trait does not enforce `Send +
+Sync`, also `abci-rs` is not easy to work with when rest of the application is using `async` Rust etc.). A decision was
+made to explore other architectures for `rust-abci` which are also compatible with `async` Rust.
+
+### Current state
+
+Currently, there are two new crates which were created in order to explore other architectures for `rust-abci`,
+`abci-rs` and `abci2`.
+
+`abci2`'s architecture is very minimal and is akin to a wrapper around raw `TcpStream` with `protobuf` encoding/decoding
+of ABCI types. All the burden of handing requests of different types in different ways is left to the developer.
+
+Unlike `abci2`, `abci-rs`' architecture is similar to `rust-abci` with some additional features, for example, support
+for [`async_trait`](https://docs.rs/async-trait)s, ABCI logic [sanity checks](https://github.com/tendermint/rust-abci/issues/49)
+with proper test coverage, support for [unix sockets](https://github.com/tendermint/rust-abci/issues/30).
+
+## Decision
+
+Absorb `abci-rs` and adopt types in `tendermint-rs` repository (using `tendermint-proto` crate, etc.).
+
+## Status
+
+Proposed
+
+## Consequences
+
+### Positive
+
+- `abci-rs` will be kept up to date with the `tendermint-rs` development and remain compatible with other related
+  crates.
+- `abci-rs` can share dependencies with other related crates (for example, simpler types from `proto`, etc.).
+- Less burden on developer/crate user.
+
+### Negative/Neutral
+
+- Developer is forced to use `async` Rust.
+- Less flexibility for the developer/crate user.
+
+## References
+
+Issues in `tendermint-rs`:
+
+- [High Level Goals for the next Third](https://github.com/informalsystems/tendermint-rs/issues/388)
+
+Issues in `rust-abci`:
+
+- [Rearchitecting Rust ABCI](https://github.com/tendermint/rust-abci/issues/61)
+- [ABCI logic "sanity checks"](https://github.com/tendermint/rust-abci/issues/49)
+- [Add support for unix sockets](https://github.com/tendermint/rust-abci/issues/30)
+
+Documentation for `abci-rs`:
+
+- [docs.rs](https://docs.rs/abci-rs/0.10.0/abci/)