Change the return values for these functions - now they return the unmarshaled result plus
the size of the unmarshaled result in bytes, so the caller could re-slice the src for further unmarshaling.
This improves performance of these functions in hot loops of VictoriaLogs a bit.
This panic could occur when samples with too long label values are ingested into VictoriaMetrics.
This could result in too long fistItem and commonPrefix values at blockHeader (up to 64kb each).
This may inflate the maximum index block size by 4 * maxIndexBlockSize.
ioutil.ReadAll is deprecated since Go1.16 - see https://tip.golang.org/doc/go1.16#ioutil
VictoriaMetrics requires at least Go1.18, so it is OK to switch from ioutil.ReadAll to io.ReadAll.
This is a follow-up for 02ca2342ab
