Upstream keepalive: clean read delayed flag in stored connections.

Upstream keepalive: clean read delayed flag in stored connections.

If a connection with the read delayed flag set was stored in the keepalive
cache, and after picking it from the cache a read timer was set on that
connection, this timer was considered a delay timer rather than a socket read
event timer as expected.  The latter timeout is usually much longer than the
former, which caused a significant delay in request processing.

The issue manifested itself with proxy_limit_rate and upstream keepalive
enabled and exists since 973ee2276300 (1.7.7) when proxy_limit_rate was
introduced.