ENG-5320: Notify table streamers of tuple insertion during load table.

Load table didn't notify table streamers that new tuples are inserted, causing
snapshot to fail due to tuple count mismatch.

Refactored both tuple insertion paths to share a common method for inserting a
tuple into the table.
ENG-5320: Fix memory access problem caused by refactoring of persistent table's
insert call.

Retain the tuple that's used to create the constraint failure exceptions.
Merge pull request #876 from VoltDB/ENG-5320

ENG-5320: Fix snapshot with loadTable

r=aweisberg