KNOX-2788 - Implementing EmptyVerifier and Cleaning background thread

[MrtnBalazs]

What changes were proposed in this pull request?

I added an empty implementation of the ConcurrentSessionVerifier (EmptyConcurrentSessionVerifier), now it is the default so users do not waste resources when they forget to configure the verifier. If they need the InMemoryConcurrentSessionVerifier implementation they need to configure it and pay attention to configure at least one user in at least one of the groups.
For example:

<property>
        <name>gateway.privileged.users</name>
        <value>admin</value>
</property>
<property>
        <name>gateway.service.concurrentsessionverifier.impl</name>
        <value>org.apache.knox.gateway.session.control.InMemoryConcurrentSessionVerifier</value>
</property>

I also added a background thread which cleans out the expired tokens from the InMemoryConcurrentSessionVerifier to save some resources. The default cleaning time is 30 minutes, but it can be configured by adding how many minutes we want the cleaning period to be:

<property>
        <name>gateway.session.verification.expired.tokens.cleaning.period</name>
        <value>80</value>
</property>
<property>
        <name>gateway.privileged.users</name>
        <value>admin</value>
</property>
<property>
        <name>gateway.service.concurrentsessionverifier.impl</name>
        <value>org.apache.knox.gateway.session.control.InMemoryConcurrentSessionVerifier</value>
</property>

How was this patch tested?

I have written unit tests for the ConcurrentSessionVeriferFactory to test that the factory gives back the right implementation for the right configuration.
I also added a unit test into InMemoryConcurrentSessionVerifierTest to test the background thread removing the expired tokens.
I also added stress tests to InMemoryConcurrentSessionVerifierTest to test whether the locking is working and the InMemoryConcurrentSessionVerifier is truly thread safe.

I also tested the behaviour with the following configurations:

1. Nothing
2. Empty String

<property>
        <name>gateway.service.concurrentsessionverifier.impl</name>
        <value></value>
</property>

3. Empty implementation

<property>
        <name>gateway.service.concurrentsessionverifier.impl</name>
        <value>org.apache.knox.gateway.session.control.EmptyConcurrentSessionVerifier</value>
</property>

4. InMemory implementation without groups configured

<property>
        <name>gateway.service.concurrentsessionverifier.impl</name>
        <value>org.apache.knox.gateway.session.control.InMemoryConcurrentSessionVerifier</value>
</property>

5. InMemory fully configured

<property>
        <name>gateway.non.privileged.users</name>
        <value>tom,guest</value>
</property>
<property>
        <name>gateway.privileged.users</name>
        <value>admin</value>
</property>
<property>
        <name>gateway.privileged.users.concurrent.session.limit</name>
        <value>2</value>
</property>
<property>
        <name>gateway.non.privileged.users.concurrent.session.limit</name>
        <value>1</value>
</property>
<property>
        <name>gateway.session.verification.expired.tokens.cleaning.period</name>
        <value>80</value>
</property>
<property>
        <name>gateway.service.concurrentsessionverifier.impl</name>
        <value>org.apache.knox.gateway.session.control.InMemoryConcurrentSessionVerifier</value>
</property>

[MrtnBalazs]

@zeroflag @smolnar82

[smolnar82]

LGTM