Add context to ShardConn errors.
basic tests for vtconn.
Use transaction id instead of flags for checking
vtconn: better sanity checks for transactions.
vtconn: more transaction & streaming tests.
vtconn: all tests complete
Simplified Barnacle protocol.

Because we have per-client sessions, there's no need to use
transaction ids for transactions.
Merge branch 'master' of github.com:youtube/vitess