Dev/issue 15 reset to default

Introduces two new functions:

     * ps_is_instrument_default_enabled(in_instrument)
     * ps_is_instrument_default_timed(in_instrument)

which returns whether the instrument is enabled/timed by default respectively.

Then these functions are used to fix issue #15 (https://github.com/MarkLeith/mysql-sys/issues/15) that disabled instruments where not enabled when resetting the Performance Schema settings to the default.