OIDs allocation as contiguous range leads to fragmentation and early failure

It is related to MCOL-5987.

If we perform CREATE TABLE/DROP TABLE like this:

1. CREATE TABLE t1000 with 1000 columns
2. CREATE TABLE t1_i with one column
3. DROP TABLE t1000
4. CREATE TABLE t1_i_plus_1 with one column

...about 17000 times we will exhaust 1000-long ranges earlier than possible and, despite use of 1/1000 of OIDs, we will be unable to create 1000-column table.

This is due to allocOIDs() returning span of free OIDs and not a set of free OIDs. In latter case free OIDs can be drawn from any free range, removing any fragmentation.