* Fix data race for max connection limiting in proxy directive.
The Conns and Unhealthy fields are updated concurrently across all active
requests. Because of this, they must use atomic operations for reads and
writes.
Prior to this change, Conns was incremented atomically, but read unsafely.
Unhealthly was updated & read unsafely. The new test
TestReverseProxyMaxConnLimit exposes this race when run with -race.
Switching to atomic operations makes the race detector happy.
* oops, remove leftover dead code.
If only one upstream is defined we don't need to buffer the body.
Instead we directly stream the body to the upstream host,
which reduces memory usage as well as latency.
Furthermore this enables different kinds of HTTP streaming
applications like gRPC for instance.
* Filter empty headers
Some web servers (e.g. Jetty 9.3) don’t like HTTP headers with empty values. This commit filters header replacements with zero length.
* Extend tests to verify removal of empty headers
* Handle add-header case
* Change - Use short variable assignment
