style: Enable rustfmt merge_imports and format
This commit enables the rustfmt merge_imports setting
and formats the whole code base accordingly.
Note that the setting is not stable yet, but will be with Rust 1.38.
In the meantime, running fmt on stable will just leave the
changes alone so no problems should occur.
The previous implementation used a futures_cpupool for executing
blocking juniper operations. This pool comes in addition to the
thread pool started by hyper through tokio for executing hyper's futures.
This patch uses tokio::blocking to perform the blocking juniper
operations while re-using the same thread pool as hyper, which
simplifies the API.
