Delete intermediate tables after use

[RasmusSkytte]

Intent

Fixes #86

Approach

• A new function defer_db_cleanup was added which uses withr::defer to remove the specified table when the function exits.
  By inserting this function into our functions when we generate new tables, the intermediate tables created will automatically be removed once the function returns.

EDIT: to clarify, it will delete the table once the parent of defer_db_cleanup exits, not when defer_db_cleanup exits.

• A new function unique_table_name() is added (inspired by dbplyrs internal function of the same name) which gives an easy way to create table names like SCDB_### for tables.
  By using this function, we can keep track of the tables that we explicitly need and which should be kept when the function returns (which we can give the "SCDB_" prefix) and those we implicitly create in intermediate computations and should not remain after the function returns (which will get the "dbplyr_" prefix).

• A new testing helper connection_clean_up() is added that is used at the end of tests that use a data base backend.
  This function looks for the existence of dbplyr_### tables and gives a warning if they exists. Then it closes the connection to the data base.

Known issues

This PR also contains the code being merged in

• Move functions to separate files #95

Once merged, this PR will be rebased and opened.

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