MB-24376: Stop rangeRead on invalid Seqno

MB-24376: Stop rangeRead on invalid Seqno

An exception is thrown if the start of the readRange is updated with a
value x < 0 || x > endSeqno, as this should not be done.

When updating an item which is currently covered by a rangeRead, the
following states occur in the seqList:

 A₁ B₂ C₃        Initial items
[A₁ B₂ C₃]       rangeRead to highSeqno

{ Write Lock

[A₁ B₂ C₃] A₋₁   ** Update; A₁ marked stale, new OSV appended.
                 NB: this is before queueDirty updates seqNo

[A₁ B₂ C₃] A₄    Seqno updated in queueDirty

}

RangeReads do not hold the write lock, and stop only upon either
reaching the end of the seqList, or reading an item with
seqno > endSeqno.
There is a brief window at the state marked ** in which a rangeRead
may try to update the start of the readRange with -1:

- rangeRead (endSeqno 3) reads C₃ normally
- proceeds to next item in the seqList
- checks if seqno > endSeqno - !(-1 > 3), don't stop yet
- update the start of the readRange with -1

This patch ends the readRange if the current item has a seqno < 0.

Change-Id: Ia1e53e4c40705b673c1f01e1b15fecbed5dff2fc
Reviewed-on: http://review.couchbase.org/78353
Tested-by: Build Bot <build@couchbase.com>
Reviewed-by: Manu Dhundi <manu@couchbase.com>
Reviewed-by: Dave Rigby <daver@couchbase.com>