Remove DnsCacheListener from DnsCache when resolver is closed - #6174

Motivation:

The RefreshingAddressResolver is added to the DnsCache as listener but is never removed from it when the resolver is closed. This makes the DnsCache hold a strong reference (internally, in the CopyOnWriteArrayList that stores all the listeners) of it even when the resolver is closed, which may cause memory leak if the DnsCache is a global one and lives for the lifetime of the JVM.

Modifications:

• Removes the DnsCacheListener from the DnsCache to make it eligible for GC.

Result:

• Closes Possible memory leak wrt RefreshingAddressResolver #6173.
• When the resolver is closed, the DnsCache no longer stores it in the list and hence no longer holds a reference of it.