OOM when importing a large database without hash index

[adsharma]

Repro:

• Download the wikidata dataset
• Run the following script:

lbug> export database 'tmp';

$ lbug tmp2.lbdb;
lbug> CALL enable_default_hash_index=false;
lbug> import database 'tmp';

Several improvements were done in #616 to improve this use case. But the OOM continues to crash on a 64GB machine.