Bug 3445: add more indexes on the action_logs table

This patch adds database indexes for action_logs table to speed up the
"log viewer" page.

Removes the existing index on timestamp+user to add an index on each
column since search colums are separately defined in log viewer form.

Test plan:
- Update database
- Play with log viewer : /cgi-bin/koha/tools/viewlog.pl
- Perform searches with only one filter defined
- Also check you see indexes with SQL query :
    SHOW CREATE TABLE action_logs

Signed-off-by: Mathieu Saby <mathieu.saby@univ-rennes2.fr>

Signed-off-by: Katrin Fischer <Katrin.Fischer.83@web.de>
Rephrased the updatedatabase message a bit:
Add indexes to action_logs table
Passes all tests and QA script.

Signed-off-by: Galen Charlton <gmc@esilibrary.com>
Bug 3445: DBRev. 3.15.00.021

Signed-off-by: Galen Charlton <gmc@esilibrary.com>