Added option 'reorderBufferAllowUpdate' to storage schemas

[fkaleo]

Added option 'reorderBufferAllowUpdate' to storage schemas.
When enabled, a point sent within the reorder buffer window that has a timestamp already used by a point in the reorder buffer will overwrite the existing value.

Fix #1531

[Dieterbe]

the reorderbuffer setting is per metrics pattern inside of storage-schemas.conf
the assumption is that different kinds of data may have different needs (some may not need ROB, others may need it, some may require a bigger buffer than others)
I think it would make sense to implement this setting the same way: users may only want to allow overwriting for certain metrics but not others.

also can we simplify the name of the setting? e.g. allow-update

[fkaleo]

the reorderbuffer setting is per metrics pattern inside of storage-schemas.conf
the assumption is that different kinds of data may have different needs (some may not need ROB, others may need it, some may require a bigger buffer than others)
I think it would make sense to implement this setting the same way: users may only want to allow overwriting for certain metrics but not others.

also can we simplify the name of the setting? e.g. allow-update

Thanks, I reworked the whole thing to address all your good remarks.

[Dieterbe]

some minor tweaks needed, LGTM