Overhaul site_stats table

Overhaul site_stats table

The site stats table holds a bunch of metric fields, two of which are of
data type "bigint unsigned", 3 are "bigint" (signed) and one is int
(signed).  Also the default values differ widely: It is 0 on the
"unsigned" fields and the "int" field, but -1 on the three others. This
patch makes all of this more consistent:

Set all fields (except the ss_row_id, which isn't changed) data type to
"bigint unsigned". Also set NULL as the default value for all those
fields. Obviously -1 isn't a possible default value any more. Also, 0
can easily be mistaken for a real value (e.g. ss_active_users=0 -->
"there is nobody active on this wiki"). NULL, by it's definition, is the
value of choice for a value to insert into fields of which we don't know
a correct value.

The respective patch files were tested locally against MySql, Sqlite,
Postgres and SQL Server 2016. Neither oracle nor the upgrade with
update.php was tested.

Bug: T56888
Change-Id: I7d42aae434852a56b6f8dd559d8a5f3bce416021