vault-agent: copy values retrieved from bolt

[tvoran]

Byte slices returned from Bolt are only valid during a transaction, and using them outside the transaction can cause an unexpected fault address panic so this makes a copy in the persistent caching functions.

Similar to the pattern in fsm.go:

vault/physical/raft/fsm.go

Lines 469 to 479 in 861454e

	err := f.db.View(func(tx *bolt.Tx) error {
	value := tx.Bucket(dataBucketName).Get([]byte(path))
	if value != nil {
	found = true
	valCopy = make([]byte, len(value))
	copy(valCopy, value)
	}
	return nil
	})
	if err != nil {