[BUG] Removal of encodeZigZag breaks compatibility with v1.1.7

[mxmauro]

MarshalInt64 and UnmarshalInt64 does not use anymore encodeZigZag to convert signed from/to unsigned counterpart.

Int64 values stored on v1.1.7 appears as doubled in v1.1.8

[kgeckhart]

We just uncovered the same issue as well causing us to downgrade, grafana/walqueue#49