btreeg: add option to avoid allocations during delete range

[arjunnair1997]

Adds a DeleteRangeReuse function which takes an input parameter with a List[T] to avoid allocating the List across calls to delete range.

[tidwall]

Makes sense to me.
I'm fine with this code overall, but I would prefer not to use the 'Unsafe' suffix. Since it doesn't use the unsafe package and appears to be pretty safe on the surface. Perhaps call it DeleteRangeReuse or something similar. The comment talking about not sharing lists between goroutines should suffice.

[arjunnair1997]

@tidwall updated

[tidwall]

LGTM!