Dynomite, Support REDIS SELECT 0 command

[maksimlikharev]

In some cases Redis clients do SELECT 0, even if no db explicitly specified.

Purpose of this PR to support SELECT 0 and return error if other value is provided.

[anujphadke]

With the change -
(env) anuj@anuj-VirtualBox:~/work/dynomite$ redis-cli -h 127.0.1.1 -p 8102
127.0.1.1:8102> SELECT 0 <<<<<<<< works now
OK
127.0.1.1:8102> SELECT 1
Error: Server closed the connection
127.0.1.1:8102> SELECT 2
Error: Server closed the connection
127.0.1.1:8102> SELECT 200000
Error: Server closed the connection
127.0.1.1:8102>

Without the change it rejected "SELECT 0"
(env) anuj@anuj-VirtualBox:~/work/dynomite$ redis-cli -h 127.0.1.1 -p 8102
127.0.1.1:8102> SELECT 0
Error: Server closed the connection
127.0.1.1:8102> SELECT 1
Error: Server closed the connection
127.0.1.1:8102> SELECT 3
Error: Server closed the connection
127.0.1.1:8102> quit;
Error: Server closed the connection
127.0.1.1:8102> quit

The code change looks fine to me.
@smukil - Can you take a final look at this change?