ACCUMULO-2934 Use jetty 8 for Accumulo 1.6.x

  This patch drops Jetty back to version 8, for the 1.6.x branches, so that
  Accumulo can continue to run in Java 6. servlet-api drops back to 3.0.1 here,
  and jetty-continuation is added as a runtime dependency.

  A few small changes were made to the EmbeddedWebServer to use the
  SelectChannelConnector and its corresponding one for SSL, instead of the newer
  ServerConnector from Jetty 9.