Clean up code

See merge request !46

* Remove unused code
* Optimize imports (group 10+ and static imports)
* Remove redundant variable assignments
* Replace explicit type parameters with <>
* Remove redundant catch blocks
* Use varargs instead of explicit array instantiation
* Weaken visibility of methods and fields
* Simplify conditions
* Do not use StringBuilder for simple concatenations
* Fix code style
* Remove invalid Javadoc

Split up and rewrite  documentation

See merge request !44

Remove context.xml Servlet Container config

See merge request !45

The README file has been split up into separate files for installation
and development. A new documentation for the upgrade process has been
added.

The following changes have been made regarding content:
* Removed outdated Vagrant section
* Replaced unnecessary session persistance section
* Use simple HTTP proxy module instead of AJP for Apache httpd
* Nginx as alternative for Apache httpd
* Use Websocket proxy which makes HTTPS configuration on the Java side
  obsolete
* Additional configuration steps for server software
* Added requirements section
* Added upgrading instructions

The context.xml currently is only used to setup Tomcat's Manager
component. A file path for session persistence is set. This
configuration is unnecessary and complicatetes the setup since Tomcat
persists sessions by default without any additional configuration
according to
https://tomcat.apache.org/tomcat-6.0-doc/config/manager.html.

Partition bulk deletes and add error handling

See merge request !43

Bulk deletes run by periodic clean ups might fail when run for the first
time or after and there are a lot of documents accumulated

Lists of documents for deletion are now partitioned into sub lists with
a maximum of 500 elements using Google Guava.

Monitoring configuration

See merge request !42

Stagemonitor and its dependencies are no longer included in builds. To
enable Stagemonitor in production, the libraries must be provided by the
host system. Additionally, the Java Servlet container needs to be setup
to load these additional dependencies for the web app.

For Tomcat 8 this is done by setting up a context config file in
conf/Catalina/localhost/<context path>.xml with the following content:

    <Context>
      <Resources className="org.apache.catalina.webresources.StandardRoot">
        <JarResources className="org.apache.catalina.webresources.DirResourceSet"
            base="<path to Stagemonitor JARs>"
            webAppMount="/WEB-INF/lib">
        </JarResources>
      </Resources>
    </Context>

See https://tomcat.apache.org/tomcat-8.0-doc/config/resources.html for
more configuration options.

Log delete and cleanup operations to database

See merge request !41

Move deletion of session related data to DAO method

See merge request !40

Questions and answers of affected sessions are now deleted.