Fix issue with resuming streaming of interleaved transactions. (#394)

This resolves a problem encountered during the transition from prefetch to
replay mode, where the transform process identifies some incomplete
transactions. This issue also arises when we interrupt 'follow' and execute
a 'follow --resume'.

The root cause is upon resumption of streaming, we may encounter a
transaction where the initial messages have LSNs less than the startpos
calculated from the latest json file.

The implemented fix involves comparing the LSNs with 'startpos' only when
resuming streaming of partially complete transactions. Once 'startpos' is
reached, comparison of LSNs for subsequent messages is discontinued.

Fixes: #373