enable TCP keepalive

[ehennum]

Set keepalive at the TCP level to maintain the enode or loadbalancer connection.

Create two new wrapper classes in com.marklogic.client.impl:

• MarkLogicSSLSocketFactory extends SSLSocketFactory
• MarkLogicSocketFactory extends SocketFactory

Each of the wrapper classes:

• Has package visibility
• Delegates to the base class
• Has a package-visible constructor that takes an instance of the base class and sets a private field
• Overrides the createSocket() factory methods to
  • Construct the socket by delegating to the factory method of the field
  • Configure the constructed socket by calling socket.setKeepAlive(true)
  • Return the configured socket

During initialization of the OkHttpClient.Builder within OkHttpServices:

• Modify the calls to clientBldr.sslSocketFactory() to pass new MarkLogicSSLSocketFactory(sslContext.getSocketFactory())
• Add an else case (where sslContext is null) that calls clientBldr.socketFactory(new MarkLogicSocketFactory(SocketFactory.getDefault()))

This change should be fully backward compatible.

See internal issues: MLAAS-2580 and RFE-4350

[ehennum]

Merged into master, ready to ship as 5.3.2