Fixes esp8266#8079

Because WiFiClientSecure inherits WiFiClient, and WiFiClientSecureCtx also
inherits WiFiClient, they both end up in the list of TCP connections that
are used for WiFiClient::stopAllExcept().  This would cause the underlying
SSL connection to be closed whenever you attempted to
stopAllExcept(WiFiClientSecure)

Fix by adding a "_owner" pointer in the WiFiClient object which points to
nullptr (default case) or to the associated upper-layer connection.
When stopping all connections except one, only look at the uppermost
connections.

Because there may be multiple copies of the WiFiClientSecure, but only
a single WiFiClientCtx, use the Ctx to check for relatedness on connections.
This will avoid closing SSL connections that were assignment copies.