Fix remove duplicate transaction rejection

[vonagam]

An attempt to fix third reported issue from #3309.

Basically, if there is initRejectFn it means a transaction is used without a container, it means that _promise is not exposed, it means that if _promise is rejected there will be an unhandled rejection, it means that we should not reject that promise.

initRejectFn will do fine by itself. If an error happens before a transactor resolved (some problem with a connection) then it will reject initPromise which is returned for knex.transaction() so it is exposed and handled easily. If an error happens after a transactor resolved, it means that initPromise is already resolved (with said transactor) and calling initRejectFn will do nothing, but it is ok since only way to get here is from executionPromise rejection and it is exposed too and it will reject only after calls to commit/rollback.

(Ideally though commit and rollback for standalone transactors should reject themselves instead of executionPromise then it will be easier to handle with async/await and no need to expose executionPromise, but that is another issue...)

(Also removed unneeded duplicate call to initRejectFn since _rejecter will call it anyway)

[vonagam]

From logs you can see three unhandled rejection less: before, after.

Two of those (1, 2) removed by this PR match error messages described in the issue. (Third is just timeout stuff, a fluctuation.)

[kibertoad]

Thanks!