Persisting a transaction is done in 2 stages:
1. TestInvoke
2. Sign and send to the network.
3. At some point the tx is persisted.
Some time passes between 1 and 3, this could lead to different GAS
costs. It is a known issue for container delete: different epoch can
have different size in bytes and thus different cost to store.
Here we introduce fixed-length encoding for integers, so that the
problem can be avoided.
Signed-off-by: Evgenii Stratonikov <e.stratonikov@yadro.com>
