[sorter] Hold records in Vec instead of a BTreeMap

[arpit-saxena]

We now just insert them one after the other in the vector. When rewind is called, the vector is sorted. Iterating is just taking elements from the vector.

Related to #191

SQLite3:

$ time sqlite3 testing/testing.db "SELECT id FROM users ORDER BY zipcode" > /dev/null

real    0m0.020s
user    0m0.013s
sys     0m0.008s

Limbo without this PR:

$ time target/release/limbo testing/testing.db "SELECT id FROM users ORDER BY zipcode" > /dev/null

real    0m0.285s
user    0m0.257s
sys     0m0.014s

Limbo with this PR:

$ time target/release/limbo testing/testing.db "SELECT id FROM users ORDER BY zipcode" > /dev/null

real    0m0.084s
user    0m0.043s
sys     0m0.032s