NoClassDefFoundError for SSLSocketImpl on non-Oracle JVM prevents proxying https remote

Description

The fix for NEXUS-6838 introduced a hard dependency on sun.security.ssl.SSLSocketImpl. JREs with alternative JSSE implementations like the IBM JRE do not ship this class.

This dependency leads to a NoClassDefFoundError on attempts to configure a proxy repository with a HTTPS URL, and thus renders the proxy repository feature unusable for https URLs on non-Oracle JVMs.

Symptoms

1. Browsing remote for proxy repositories with https remote does not work
2. Logfile shows

SEVERE: Unhandled exception or error intercepted
java.lang.NoClassDefFoundError: sun/security/ssl/SSLSocketImpl
        at org.sonatype.nexus.apachehttpclient.NexusSSLConnectionSocketFactory.connectSocket(NexusSSLConnectionSocketFactory.java:114)
        at org.apache.http.impl.conn.HttpClientConnectionOperator.connect(HttpClientConnectionOperator.java:123)
        at org.apache.http.impl.conn.PoolingHttpClientConnectionManager.connect(PoolingHttpClientConnectionManager.java:318)
        at org.apache.http.impl.execchain.MainClientExec.establishRoute(MainClientExec.java:363)
        at org.apache.http.impl.execchain.MainClientExec.execute(MainClientExec.java:219)
        at org.apache.http.impl.execchain.ProtocolExec.execute(ProtocolExec.java:195)

Steps to reproduce:

1. download and install IBM JRE
2. start nexus 2.x with IBM JRE
3. configure a proxy repository with a https remote
4. navigate to "browse remote" and try to browse the content

Suggested fix

Use the plattform independent way to set the host for SNI as documented by Oracle: https://docs.oracle.com/javase/8/docs/technotes/guides/security/jsse/JSSERefGuide.html#SNIExamples

Quick fix

Remove dependency on Oracle JRE by using reflection to access SSLSocketImpl.

The quick fix makes https usable for non-Oracle JREs, but removes SNI support for these JREs.

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The quick was implemented in this issue - SNI still does not work for any JVM other than Oracle JVMs. See NEXUS-6844.
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