This is a performance improvement for webhook bridge.
Since this bridge is called using `async` callback mode, and `ehttpc`
frequently returns errors of the form `normal` and `{shutdown,
normal}` that are retried "for free" by `ehttpc`, we add this behavior
to async requests as well. Other errors are retried too, but they are
not "free": 3 attempts are made at a maximum.
This is important because, when using buffer workers, we should avoid
making them enter the `blocked` state, since that halts all progress
and makes throughput plummet.
When using async mode with the webhook bridge, queued messages that are
not fully processed when the connection times out could be lost. This
commit fixes this by letting the bridge return a recoverable_error when
this happen. The message send will then be retried in sync mode by the
emqx_resource_buffer_worker.
Fixes: https://emqx.atlassian.net/browse/EMQX-8974
Thales Macedo Garitezi