[Feat] Make eXist-db compatible with Java16

[dizzzz]

At this moment the eXist-db codebase is not compatible with Java16; Some highlights

• Jacoco crashes (SIGSEGV)
• Mocking does not work
• misc (related?) test issues

some inspiration:

[ERROR] org.exist.storage.journal.JournalXmlTest.replaceDifferentContentThenDelete Time elapsed: 0.544 s <<< ERROR!
java.lang.reflect.InaccessibleObjectException: Unable to make private native java.lang.reflect.Field[] java.lang.Class.getDeclaredFields0(boolean) accessible: module java.base does not “opens java.lang” to unnamed module @5342c36b

[ERROR] Tests run: 2, Failures: 0, Errors: 2, Skipped: 0, Time elapsed: 0.62 s <<< FAILURE! - in org.exist.xquery.XQueryContextTest
[ERROR] org.exist.xquery.XQueryContextTest.prepareForExecution_setsUserFromSession Time elapsed: 0.612 s <<< ERROR!
java.lang.ExceptionInInitializerError
	at org.exist.xquery.XQueryContextTest.prepareForExecution_setsUserFromSession(XQueryContextTest.java:51)
Caused by: org.easymock.cglib.core.CodeGenerationException: java.lang.reflect.InaccessibleObjectException-->Unable to make protected final java.lang.Class java.lang.ClassLoader.defineClass(java.lang.String,byte[],int,int,java.security.ProtectionDomain) throws java.lang.ClassFormatError accessible: module java.base does not “opens java.lang” to unnamed module @5342c36b
	at org.exist.xquery.XQueryContextTest.prepareForExecution_setsUserFromSession(XQueryContextTest.java:51)
Caused by: java.lang.reflect.InaccessibleObjectException: Unable to make protected final java.lang.Class java.lang.ClassLoader.defineClass(java.lang.String,byte[],int,int,java.security.ProtectionDomain) throws java.lang.ClassFormatError accessible: module java.base does not “opens java.lang” to unnamed module @5342c36b
	at org.exist.xquery.XQueryContextTest.prepareForExecution_setsUserFromSession(XQueryContextTest.java:51)
[ERROR] org.exist.xquery.XQueryContextTest.cleanUp_BinaryValueInstances Time elapsed: 0.001 s <<< ERROR!
java.lang.NoClassDefFoundError: Could not initialize class org.easymock.internal.ClassProxyFactory$2
	at org.exist.xquery.XQueryContextTest.cleanUp_BinaryValueInstances(XQueryContextTest.java:78)

[ERROR] org.exist.storage.blob.BlobStoreRecoveryTest.addCommit_removeCommit[shutdown] Time elapsed: 0.107 s <<< ERROR!
java.lang.NoClassDefFoundError: Could not initialize class org.easymock.internal.ClassProxyFactory$2
	at org.exist.storage.blob.BlobStoreRecoveryTest.newBlobDb(BlobStoreRecoveryTest.java:474)
	at org.exist.storage.blob.BlobStoreRecoveryTest.addCommit_removeCommit(BlobStoreRecoveryTest.java:211)

hs_err_pid25774.log
hs_err_pid27653.log