Support streaming in tsgrpc kvstore driver.

[zlobober]

This commit changes tsgrpc protocol in backward-incompatible manner, addressing issue #137. After this change, it becomes possible to read and write values that exceed the theoretic maximum of 2 GiB per proto message.

[zlobober]

@laramiel, would you mind having a look?

[laramiel]

Looking; generally looks reasonable, will try to give it more of a review tomorrow, or early next week.

[laramiel]

If you sync this, then I can look at applying it.

[zlobober]

Was on conference/sick leave, sorry for long time without response. Actually I am working on fixing everything right now, will bring an update later today/tomorrow. Thanks!