accounts: copy bbolt values before closing read transaction

bucket.Get returns a byte slice that points directly into bbolt's
mmap'd file. This slice is only valid for the lifetime of the
transaction. In Account() and LastIndexes(), the slices were escaping
the View callback and being read after the transaction closed. Under
concurrent write transactions the file gets remapped, invalidating
the pointer and causing a segmentation fault.

Copy the byte slices inside the transaction so deserialization
operates on stable, heap-allocated memory.

accounts/store_kvdb.go

@@ -457,11 +457,17 @@ func (s *BoltStore) Account(_ context.Context, id AccountID) (
 			return ErrAccountBucketNotFound
 		}
 
-		accountBinary = bucket.Get(id[:])
-		if len(accountBinary) == 0 {
+		v := bucket.Get(id[:])
+		if len(v) == 0 {
 			return ErrAccNotFound
 		}
 
+		// Copy the value since bbolt's Get returns a slice into
+		// the mmap'd file that is only valid for the lifetime of
+		// the transaction.
+		accountBinary = make([]byte, len(v))
+		copy(accountBinary, v)
+
 		return nil
 	}, func() {
 		accountBinary = nil
@@ -560,16 +566,24 @@ func (s *BoltStore) LastIndexes(_ context.Context) (uint64, uint64, error) {
 			return ErrAccountBucketNotFound
 		}
 
-		addValue = bucket.Get(lastAddIndexKey)
-		if len(addValue) == 0 {
+		av := bucket.Get(lastAddIndexKey)
+		if len(av) == 0 {
 			return ErrNoInvoiceIndexKnown
 		}
 
-		settleValue = bucket.Get(lastSettleIndexKey)
-		if len(settleValue) == 0 {
+		sv := bucket.Get(lastSettleIndexKey)
+		if len(sv) == 0 {
 			return ErrNoInvoiceIndexKnown
 		}
 
+		// Copy values since bbolt's Get returns slices into the
+		// mmap'd file, only valid within the transaction.
+		addValue = make([]byte, len(av))
+		copy(addValue, av)
+
+		settleValue = make([]byte, len(sv))
+		copy(settleValue, sv)
+
 		return nil
 	}, func() {
 		addValue, settleValue = nil, nil