Introduce java.util.Logger

[jankotek]

JDBM will probably be used in enterprise environment. We need way to collect debug info, warn users and report errors. So we should start using java.util.Logger.

There are a few traps in what inexperienced user could fall into. For example data import with transactions enabled. This would generate traffic in support group which could be avoided. So JDBM should sense such situation and log an warning: "Looks like you are doing batch-import. Try 'DB.disableTransactions()' to speed up this."

Debug and trace statements should be controlled by static final boolean field, so JDBM could be compilable without debug code.

[jankotek]

Closing, not going to fix right now