Fix the MySQL backend for the current schema

backend_mysql_load_data:
- Import coverart tables in a similar fashion to what we currently do with statistics tables.

backend_mysql_update_schema_from_file:
- Support CREATE TABLE queries that doesn't end with "\n);"
- Basic support for Postgres POINT fields (dummy mapping to VARCHAR(100))
- Map Postgres TIMESTAMPTZ datatype to MySQL TIMESTAMP.
- [Debug] Print what column is currently being added
- Ugly hack for ignoring CONSTRAINTs on CREATE TABLE that span multiple lines (checking if second word is "OR"/"IS"/"=", or if parsing yield a bracket as a field name).

I this the following issues can be closed, either because this this pull request fixes them, or because they were previously fixed but left open:
- https://github.com/elliotchance/mbzdb/issues/59 - Another schema issue: release group cover art
- https://github.com/elliotchance/mbzdb/issues/49 - MusicBrainz Schema seems to have changed (statistic tables)
- https://github.com/elliotchance/mbzdb/issues/43 - "Install/Update database schematic" table creation error. (problems with the enum types, this was already closed).
- https://github.com/elliotchance/mbzdb/issues/39 - init.pl option "create index" failed (now when a UNIQUE index creation fails, we retry with a non-UNIQUE index).
- https://github.com/elliotchance/mbzdb/issues/38 - load_data also tries to load data from directories (a patch was already merged).