Remove the socket impl factory mechanism

This issue proposes to deprecate, and eventually remove, the API points
for statically configuring a system-wide factory for the socket types in
the java.net package. Specifically, the following:

 * Methods:
  1) static void ServerSocket.setSocketFactory​(SocketImplFactory fac)
  2) static void Socket.setSocketImplFactory​(SocketImplFactory fac)
  3) static void DatagramSocket.setDatagramSocketImplFactory​(DatagramSocketImplFactory fac)

 * Types
  a) java.net SocketImplFactory
  b) java.net DatagramSocketImplFactory

The java.net SocketImplFactory and DatagramSocketImplFactory types can
be deprecated too, since their only use relates to the above factory
setter methods.

The CSR for JDK-8220494 contains some verbiage about the potential
issues that setting factories can have, and alludes to their possible
future removal.