buffer: replace linkerd-buffer with tower::buffer from upstream

[hawkw]

The proxy currently has its own implementation of a tower Service
that makes an inner service Cloneable by driving it in a spawned task
and buffering requests on a channel. This also exists upstream, as
tower::buffer.

We implemented our own version for a couple of reasons: to avoid an
upstream issue where memory was leaked when a buffered request was
cancelled, and to implement an idle timeout when the buffered service
has been unready for too long. However, it's no longer necessary to
reimplement our own buffer service for these reasons: the upstream bug
was fixed in tower 0.4 (see tower-rs/tower#476, tower-rs/tower#480,
and tower-rs/tower#556); and we no longer actually use the buffer idle
timeout (instead, we idle out unresponsive services with the separate
Failfast middleware, note that push_spawn_buffer_with_idle_timeout
is never actually used).

Therefore, we can remove our sui generis implementation in favour of
tower::buffer from upstream. This eliminates dead code for the idle
timeout, which we never actually use, and reduces duplication (since
tonic uses tower::buffer internally, its code is already compiled
into the proxy). It also reduces the amount of code I'm personally
responsible for maintaining in two separate places ;)

Since the linkerd-buffer crate erases the type of the buffered
service, while tower::buffer does not, I've changed the
push_spawn_buffer/spawn_buffer helpers to also include a
BoxService layer. This required adding a BoxServiceLayer type, since
BoxService::layer returns a LayerFn with an unnameable type.

Also, this change ran into issues due to a compiler bug where generators
(async blocks) sometimes forget concrete lifetimes,
rust-lang/rust#64552. In order to resolve this, I had to remove the
outermost async blocks from the OpenCensus and identity daemon tasks.
These async blocks were used only for emitting a tracing event when the
task is started, so it wasn't a big deal to remove them; I moved the
trace events into the actual daemon task functions, and used a tracing
span to propagate the remote addresses which aren't known inside the
daemon task functions.

[olix0r]

this looks great! a few nits...

[olix0r]

nit: sort

[olix0r]

tioli: consider adding to svc:

type Buffer<Req, Rsp> = buffer::Buffer<BoxService<Req, Rsp, Error>, Req>;

to simplify references like this

[hawkw]

yeah, good call

[olix0r]

rm?