oops
disable sequential skipahead

this was causing some problems on very-many-core systems

still need to update operation flags and disable seq mode for
insert/extract/etc.
remove support for seq mode read for extract etc.

see comment in sequential

We used to not stall if the read position was zero, ie. if the first
request was for a line some way down the image, and assume this was
extract or somesuch. But this could sometimes break on busy, many-core
systems.

Think of a better way to support eg.  extract safely in sequential mode.
Merge remote-tracking branch 'origin/7.38'
Merge remote-tracking branch 'origin/7.38'
notes