grafana#1760 noted that creating a new rand.Rand every time a
random number is generated causes a poor distrubution of the search
space, where numbers appear 7x more frequently than they do when using a
shared rand.Rand instance.

This is demonstrated using the following Go playground link: https://go.dev/play/p/EkhINRfOO5p

This commit changes Vulture to use a shared rand.Rand instance instead
of creating a new one each time a search or read is performed.