Cache list of open channels locally

[vforvalerio87]

Current behavior:
When a service client instance is created, the list of channels for that service is fetched from the blockchain event log from the block where the MultiPartyEscrow was deployed for that network up to the latest block

Desired:
The list of channels from the blockchain can be cached locally until the last analyzed block and then re-fetched from the next block to the latest. Different client instances can leverage the same list of channels

[astroseger]

related to #9 (we should unify with snet-cli there it is already implemented ).