executor: switch from `log` to `tracing` (#1454)

## Motivation

The `tracing` crate implements scoped, structured, context-aware
diagnostics, which can add significant debugging value over unstructured
log messages. `tracing` is part of the Tokio project. As part of the
`tokio` 0.2 changes, I thought it would be good to move over from `log`
to `tracing` in the tokio runtime. Updating the executor crate is an obvious
starting point. 

## Solution

This branch replaces the use of `log` in `tokio-executor` with
`tracing`. I've tried to leave all the instrumentation points more or
less the same, but modified to use structured fields instead of string
interpolation. I've also added a few `tracing` spans, primarily in
places where a variable is added to all the log messages in a scope.

## Notes

For users who are using the legacy `log` output, there is a feature flag
to enable `log` support in `tracing`. I thought about making this on by
default, but that would also enable the `tracing` dependency by default,
and it is only pulled in when the `threadpool` feature flag is enabled.
The `tokio` crate could enable the log feature in its default features
instead, since the threadpool feature is on by default in `tokio`. If
this isn't the right approach, I can change how `log` back-compatibility
is enabled.

We might want to consider adding more `tracing` spans in the threadpool
later. This could be useful for profiling, and for helping users debug
the way their applications interact with the executor. This branch is
just intended as a starting point so that we can begin emitting
`tracing` data from the executor; we should revisit what instrumentation
should be exposed, as well.

Signed-off-by: Eliza Weisman <eliza@buoyant.io>

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