Correctly update SYS_COLUMNS

innobase_instant_try(): If any columns were dropped or
reordered, delete any SYS_COLUMNS records for the following
columns, and insert SYS_COLUMNS records for all subsequent
stored columns as well as for all virtual columns.

commit_try_norebuild(): If ctx->is_instant(), let the virtual
columns be added or dropped by innobase_instant_try().