MAINT, DOC: Cleanup, comments, documentation added.

src/main/java/caosdb/datetime/UTCDateTime.java

.../backend/implementation/MySQL/DatabaseConnectionPool.java

...er/database/backend/implementation/MySQL/MySQLHelper.java

...backend/implementation/MySQL/MySQLUpdateSparseEntity.java

.../server/database/backend/transaction/RetrieveParents.java

...ver/database/backend/transaction/RetrieveVersionInfo.java

src/main/java/caosdb/server/datatype/ReferenceValue.java

src/main/java/caosdb/server/entity/Entity.java

src/main/java/caosdb/server/entity/xml/SetFieldStrategy.java

...main/java/caosdb/server/transaction/WriteTransaction.java