Resolve "MySQL schema support for lease limits"

Closes #2438 (closed).

• 5d2d52bf some outstanding minor corrections
• 1b4a29f1 MySQL schema changes in support of lease limiting
• 4c74672d add tests for lease counting
• 97d730f9 add ChangeLog entry

Apart from the minor corrections mentioned above, justifying some changes:

• While developing these schema changes, lease upload tests would fail because of the schema changes in this MR. On a second run, it would halt because of kea-admin asking for input. The changes in lease dump and lease upload tests address this problem. The tests clean up better now, removing /tmp/$(basename "${output_file}").tmp or "/tmp/$(basename "${input_file}").tmp" based on the test that is run. You can see these files are prompted input on by searching check_file_overwrite in kea-admin.in.
• The truncation in are-scripts-in-sync.py is no longer required since we started matching specific versions for the This.*starts line. And it causes the script to display not relevant output if you have a difference that spans too many lines. You can test it by removing a sizable portion from dhcpdb_create or the upgrade script. So the truncation was removed.
• The change from This line starts the database upgrade to This line starts the schema upgrade is purely cosmetical.
• Some UPGRADE schema_version statements were missing in dhcpdb_create.pgsql. They are important for debugging what version you're on in case the script fails.