connector-c-shapshot: support snapshot file format by client library:

* xlog/snapshot file format iterator object added (tnt_log)
* tnt_snapshot stream created
* refactored tnt_xlog and tnt_stream streams to use the same tnt_log object
* storage iterator added (for fetching tuples from snapshot or other
tuple storage)
* console client refactored to support snapshot files (cat command)
(this also fix for https://bugs.launchpad.net/tarantool/+bug/1028475)
* snapshot test case added