Conditionally extend MAX_RANKs to 4-byte integer range for PostgreSQL

[bnauta]

MAX_RANK_VALUE and MIN_RANK_VALUE are set to MySQL's 3-byte MEDIUMINT range by default. However, as PostgreSQL doesn't have a 3-byte integer data type and Postgres users are forced into allocating a 4-byte integer anyway, we may as well use the full 4-byte range to reduce the chance of potentially expensive rebalancing.

For reference:
Signed 4-byte INTEGER in PostgreSQL: -2147483648 to +2147483647
Signed 3-byte MEDIUMINT in MySQL: -8388607 to +8388607
SQLite3 allocates space according to size of each value, so we'll stick with the defaults.

[mixonic]

Closed in favor of #109. @bnauta sorry there is so much effort that was set aside after this, but thank you for the contribution

[bnauta]

@mixonic np!