plugin usage numbers in the feedback reports

We want to see what plugins are used and what aren't. To profile this, we need plugin usage numbers in the feedback plugin reports.

An easy way to get a number that approximately corresponds to how often a plugin was used would be to count how many times a plugin was locked.

Adding it to I_S.FEEDBACK table reveals another problem. Currently it reports plugin versions as, say,

...

| Xxx | 1.2 |

...

for plugin "Xxx". To report usage counters we'll add another row like

...

| Xxx used | 1234 |

...

with the space in the name, to avoid possible clashes with anything from the show status or show variables.

For consistency, we'll report versions as

...

| Xxx version | 1.2 |

...