Get socket timeout once per command execution

[chapsuk]

Hi, this fix was born from error log message (caller string):

command execution timed out on client: Exceeded number of retries. 
See `Policy.MaxRetries`. (last error: %!s(<nil>))

%!s(<nil>) because err var redefined here for each loop iteration.

Looks like socketTimeout can't change between execution iterations and we can define him once.

With this fix resulting error message looks like this:

command execution timed out on client: Exceeded number of retries. 
See `Policy.MaxRetries`. (last error: No available connections to the node. 
Connection Pool was empty, and limited to certain number of connections.)