Offer selectable strategies for local caching of channel state

[vforvalerio87]

Current behavior:
Right now channel state is re-fetched from the daemon at every call: the reason is that another application using the same private key and channel (example: dapp or CLI) may update the state of that channel for that user and the sdk wouldn't know about it

Desired behavior:
Allow users to specify pluggable caching strategies for channel state. Example: store in a variable for the current session, store to local file system, store to shared file system or database (this way multiple SDK instances could share the same channel)

[astroseger]

directly related #9 . It is already implemented in snet-cli (together with dynamical caching for metadata and compiled protobufs).