esp_http_server : Only accept new connections if server has capacity to handle more

esp_http_server : Only accept new connections if server has capacity to handle more

This fix prevents HTTP server from accepting new connections when the total count of connected
sockets has reached the max_open_sockets limit set during configuration. The pending connections
are kept in backlog until atleast one of the connected sockets is closed. The maximum number of
connection requests that can kept in backlog is specified as backlog_conn configuration option.
Note that this modification has no effect when LRU purge is enabled.

Also added sanity check on setting for max_open_sockets during configuration.

Solution suggested by jimparis https://github.com/espressif/esp-idf/issues/3183#issue-421234265

Closes https://github.com/espressif/esp-idf/issues/3183