Renaming funcitons for future 1.0.0 release

Function	Suggestion	Reason
`defer_db_cleanup`	`defer_remove_table`	`defer_` works well with `withr` convention but `db_cleanup` is not clear. `remove_table` is aligned with `DBI::dbRemoveTable`.
`is.historical`	`is_historical`	Periods in functions are a confusing relic of R which interacts badly with S3 methods
`digest_to_checksum`	`add_checksum`	Since we are returning the data with a `checksum` column added or mutated if existing so `mutate_checksum` could be an alternative
`update_snapshot`	`save_snapshot`	I don't think were "updating" snapshots rather, we are saving snapshots

Function	Comment
`filter_keys`	`filter_keys` works okay-ish, but maybe `subset` gives a more clear picture. It is more a filter than a subset - as applied to both target and incoming.
`interlace`	I think `interlace` is not bad, but like other alternatives in my mind, it does not exactly describe
`unique_table_name`	Not clear if it makes or checks uniqueness.
`Logger`, `LoggerNull`	Uses R6 naming scheme (PascalCase) which fits well with convention
`get_connection`, `close_connection`	Clear concise names
`create_table`, `create_index`, `create_logs_if_missing`	Clear concise names
`get_catalog`, `get_schema`, `get_table`, `get_tables`	Clear concise names
`lock_table`, `unlock_table`	Clear concise names
`id`	Inspired by the `DBI::Id()` function
`slice_time`	Follows the naming style of `dplyr::slice_min()`, `dplyr::slice_max()` etc.
`table_exists`, `schema_exists`	Clear concise names
`db_timestamp`	Good enough

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