Remove the per-instance logs query cache

Use only the `wp_cache`.

Fixes #144
Add scope attribute to User column in logs table

See #98
Add a label to the points logs search form

For improved accessibility.

See #98
Only call _prepare_query() in _get_sql()

Removes duplicate code in `WordPoints_Points_Logs_Query`.

During work on #115
Avoid an undefined offset notice

Introduced in work on #114
Put space after comma, not before

Discovered while working on #115
Allow points logs query arguments to modified

Fixes #115