disconnect if selection of db fails

[alex-67]

tornadis always uses the default db0 after redis-server restarts regardless of the db number which was set during tornadis client initialization. This occurs when redis-server version is lower than 2.6.

"select" operation doesn't work when redis is loading the dataset. redis/redis#1024

[coveralls]

Coverage increased (+0.01%) to 90.748% when pulling 4a25225 on alex-67:select-db-fix into f2cf630 on thefab:master.