Proxies may buffer TCP connections while waiting for policy discovery
and may buffer HTTP requests while waiting for a shared resource (like a
load balancer).

Previously, a single configuration was used to configure both TCP and
HTTP buffers. This change decouples these configurations in preparation
for allowing balancer configuration to be configured by the control
plane.

Furthermore, this change updates the stack builder to always construct
buffers with failfast and spawnready to (1) ensure that all buffers
enforce the proper backpressure and load shedding semantics and (2) to
reduce boilerplate.

This change updates the proxy's environment configuration as follows:

* Removed `LINKERD2_PROXY_INBOUND_DISPATCH_TIMEOUT`
* Removed `LINKERD2_PROXY_OUTBOUND_DISPATCH_TIMEOUT`
* Added `LINKERD2_PROXY_INBOUND_DISCOVERY_IDLE_TIMEOUT`
* Added `LINKERD2_PROXY_OUTBOUND_DISCOVERY_IDLE_TIMEOUT`
* Removed `LINKERD2_PROXY_BUFFER_CAPACITY`
* Removed `LINKERD2_PROXY_INBOUND_ROUTER_MAX_IDLE_AGE`
* Removed `LINKERD2_PROXY_OUTBOUND_ROUTER_MAX_IDLE_AGE`
* Added `LINKERD2_PROXY_INBOUND_HTTP_BUFFER_CAPACITY`
* Added `LINKERD2_PROXY_INBOUND_HTTP_FAILFAST_TIMEOUT`
* Added `LINKERD2_PROXY_OUTBOUND_TCP_BUFFER_CAPACITY`
* Added `LINKERD2_PROXY_OUTBOUND_TCP_FAILFAST_TIMEOUT`
* Added `LINKERD2_PROXY_OUTBOUND_HTTP_BUFFER_CAPACITY`
* Added `LINKERD2_PROXY_OUTBOUND_HTTP_FAILFAST_TIMEOUT`
* Added `LINKERD2_PROXY_OUTBOUND_HTTP1_CONNECTION_POOL_IDLE_TIMEOUT`