Node should re-use instances of hash.Hash by using a sync.Pool

[fasmat]

Description

This issue came up in a discussion here: #5784 (comment)

Every time the node hashes data it re-instantiates hash.Hash objects, which is slow and can be mitigated by using a sync.Pool.

Affected code

An example would be here:

go-spacemesh/common/types/wire/activation_v1.go

Line 129 in 28480f5

h := hash.New()

Other places where instances of hash are created should also be checked to be re-used.