RFR: 8235139: Remove the socket impl factory mechanism

[Patrick Concannon]

Hi,

Could someone please review my proposed changeset for JDK-8235139: '`Remove the socket impl factory mechanism`' ?

These changes propose to deprecate (for the eventual removal) the API points for statically configuring a system-wide factory for the `Socket`, `ServerSocket`, and `DatagramSocket` types in the `java.net package`. Specifically, the following:

**Methods**:
- `static void Socket.setSocketImplFactory​(SocketImplFactory fac)`
- `static void ServerSocket.setSocketFactory​(SocketImplFactory fac)`
- `static void DatagramSocket.setDatagramSocketImplFactory​(DatagramSocketImplFactory fac)`

**Types**:
- `java.net SocketImplFactory`
- `java.net DatagramSocketImplFactory`

The CSR for[ JDK-8220494](https://bugs.openjdk.java.net/browse/JDK-8220494) contains some verbiage about the potential
issues that setting factories can have, and alludes to their possible
future removal. 

Kind regards,
Patrick

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Commit messages:
 - 8235139: Remove the socket impl factory mechanism

Changes: https://git.openjdk.java.net/jdk/pull/2375/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=2375&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8235139
  Stats: 53 lines in 3 files changed: 26 ins; 22 del; 5 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2375.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2375/head:pull/2375

PR: https://git.openjdk.java.net/jdk/pull/2375