Upstream: enabled keepalive by default for explicit upstreams.

Upstream: enabled keepalive by default for explicit upstreams.

Keepalive is now automatically enabled in the "local" mode for upstreams
defined in configuration files. Cached keepalive connections are no longer
shared between different locations referencing the same explicit upstream
unless keepalive is explicitly configured without the "local" parameter.

To disable keepalive entirely, use keepalive 0; inside the upstream block.
To allow sharing cached connections between locations, configure
keepalive <max_cached>; without the "local" parameter.