MemoryDB: Building non-unique indexes is nondeterministic

[nicktobey]

Repro:

create table test(c0 int not null, c1 int not null, pk int primary key, key (c0, c1));
insert into test values (2, 3, 1), (5, 6, 4), (2, 3, 7);",
select * from test where c0 = 2 and c1 = 3;

The iteration order of the select varies each time the index is built. Usually the row (2, 3, 1) is returned first, but occasionally the row (2, 3, 7) is returned first instead.

I imagine this is happening because we're iterating over a golang map at some point during index building.

This can cause issues because it can lead to the memory DB having different behavior from Dolt, which complicates writing Dolt tests.