Option to skip merge and direct copy for insert only tables

[alok87]

Some huge tables are insert only tables, you do not perform UPDATES or delete on it. Such tables are huge transaction tables or events tables. User should have option to specify these tables can skip merge and directly load data.

Such huge tables spend major time in doing the delete-common operations, 1.3minutes in the above one while it takes only few seconds to copy the data.

If we can skip merge for such tables it would help speed up the loads and free up the queue for other to load fast.

Should help in #186

This was tried in past and reverted https://github.com/practo/tipoca-stream/pull/88/files later