Replace store_id int(11) by store_id SMALLINT(5) UNSIGNED

SumoStore currently has a 4 byte `INT` for the store ID, which allows
for up to 4,294,967,296 stores. This is sheer overkill. Even a large
corporation like McDonalds’s only has about 35,000 outlets. Reducing the
store ID to an unsigned `SMALLINT` saves 2 bytes and will still allow
for up to 65,536 stores.