Improve performance of load_table()

[jamesaoverton]

Anecdotally, I'm finding rltbl load to be slower than I expect. There are two things that I think will help:

1. Wait to CREATE TRIGGER until after INSERT
2. INSERT multiple rows at a time, limited by the number of parameters allows in a SQLite query (see Splitup code #9 https://www.sqlite.org/limits.html)

[lmcmicu]

Just a small note regarding 1: In the case where we are asked to load an already existing table, we should add a drop trigger if exists statement before the create table if not exists statement.