TLS foundation: tls-channel dep + TlsContextFactory

Adds the marianobarrios:tls-channel:1.0.0 dependency (thin ByteChannel-over-
SSLEngine wrapper, MIT, zero transitive deps, Java 8 bytecode) and a
TlsContextFactory that builds server (keystore) and client (pinned truststore)
SSLContexts, TLS 1.3 only. No transport wiring yet — this just compiles in the
building blocks for full-session encryption of the Presents NIO stream.

core/pom.xml

@@ -23,6 +23,14 @@
       <artifactId>guice</artifactId>
       <version>5.1.0</version>
     </dependency>
+    <dependency>
+      <!-- thin ByteChannel-over-SSLEngine wrapper used to give the Presents NIO transport
+           full-session TLS (see com.threerings.presents.net.TlsContextFactory); MIT, no
+           transitive deps, targets Java 8 -->
+      <groupId>com.github.marianobarrios</groupId>
+      <artifactId>tls-channel</artifactId>
+      <version>1.0.0</version>
+    </dependency>
 
     <!-- optional dependencies -->
     <dependency>