Truncate tables when running silk_clear_request_log

[albertyw]

Fixes #239

This should make ./manage.py silk_clear_request_log run a lot faster by using TRUNCATE for non-sqlite databases. I believe that objects.all().delete() won't run TRUNCATE on these tables because of foreign key checks.

[AuHau]

Just small comment regarding the FKs.

In Postgres you can truncate multiple tables with one command and if you specify all tables which have the FK dependencies just between each other (eq. all Silk tables), then you don't have to turn off the FK checks ;-)

I have been using this for some time:
TRUNCATE TABLE silk_profile, silk_response, silk_profile_queries, silk_request, silk_sqlquery;

I checked Postgres documentation and multiple-table truncation is possible since 8.1 version, so it should be safe ;-)

Unfortunately this is not possible in MySQL :-/

[AuHau]

Hmmm I just saw how the whole command is implemented and without bigger rewrite the previously mentioned technique is not really usable :-/ Damn... Well then it is just FYI :-D

But thanks for the PR! Lets hope it will be merged...

[albertyw]

Merging this for now. This should probably be refactored and tested later.