Speedup insert/update member table.

The public key is no longer UNIQUE in the database.
Removed systemtap debug functions.
Database upgrade 17 -> 18.

The public_key column of the member table is no longer UNIQUE.  This
removes the index on this column, reducing the bytes written when new
Member instances are created.  The python code is now responsible to
ensure that public_key remains unique.
Merge pull request #127 from boudewijn-tribler/get_member-variant-3

Get member variant 3