perf(core): implement bulk deletes

[B4nan]

UoW will now group change sets based on the type and entity. Instead of ordering
all the change sets based on the type and entity, we can now travers those groups
based on the commit order.

em.remove(book1);
em.remove(book2);
em.remove(book3);

// flushing will fire only one delete query like `delete from book where id in (1, 2, 3)`
await em.flush();

Related: #732