Use proper read lock for FieldTrie

[terencechain]

In FieldReferencesCount(), we call f.Refs() which uses the reference lock

Then it calls len(f.fieldLayers) which uses the reference lock again. Instead it should be using FieldTrie lock.

// FieldTrie is the representation of the representative trie of the particular field.
type FieldTrie struct {
	*sync.RWMutex // <- We should use this lock
	*reference
	fieldLayers [][]*[32]byte // <- When reading this
	field       fieldIndex
}

type reference struct {
	refs uint
	lock sync.RWMutex // <- We SHOULD NOT use this lock
}

Note that I changed FieldTrie's lock to RWMutex to gain read locking

[codecov]

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[nisdas]

Nice Catch !