Fix data duplicate and data loss

[chaoqin-li1123]

Motivation

When start offset is exclusive and equal to end offset, current code will read past the end, which results in data duplicate.
Reader seek() is redundant inside pulsar RDD(because start offset is already specified for the reader) and sometimes cause data loss(sometimes the cursor forward when seeking the current offset).

Modifications

Remove redundant reader seek()
Set isLast flag to true if start offset = end offset when start offset is exclusive.

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