Add Logger$finalize()

[marcusmunch]

Intent

This closes #64.

Approach

I have added a Logger$finalize() function (see also R6#finalizer in Advanced R).
Currently, it does nothing unless log_path is NULL and log_tbl is not NULL.
The finalizer generates a query which sets all log_file in the DB to NULL if they were previously equal to self$log_filename (i.e. exactly what I described in #64).¹

For usability/readability, I added a bit of logic to table_exists to compensate for the shortcomings of DBI::dbExistsTable.

Known issues

More feature than bud: Since the DB connection is stored within a tbl_dbi object, table_exists does not care about the conn argument if db_table_id inherits from tbl_dbi.

Checklist

• The PR passes all local unit tests
• I have documented any new features introduced
• If the PR adds a new feature, please add an entry in NEWS.md
• A reviewer is assigned to this PR

Footnotes

1. This is done with DBI::dbExecute as dplyr::rows_update (naturally) cannot match a log_file that is not NULL with one that is 🙃 . ↩