Fix restoring velocity in software (ascanc) continuous scans

8815c914 introduced a hack to avoid recursive decrease of velocity in continuous scans. Most probably this hack was needed to avoid some synchronization problems between the software continuous scan threads (motion and acquisition are done in separate threads). When using Taurus 4 this hack is not applied cause the attribute's range returns -inf and inf floats instead of "Not specified" string and we don't fall into the except clause anymore. Refactor the CScan.get_max_top_velocity() to consider only Taurus 4 and above case (we don't support Taurus < 4 anymore) and maintain the hack.

Also, log failed setting of maximum top velocity in continuous scans.

Fixes #1574 (closed).