Support set session settings

[guoxiaod]

We want to use some clickhouse experimental features, such as window function, but we can not set session settings in current version.

1. sql
   set allow_experimental_window_functions = 1;
2. or add settings in clickhouse.OpenDB ( clickhouse.Options ):

+               Settings: clickhouse.Settings{
+                       "allow_experimental_window_functions": 1,
+               },

[mshustov]

ClickHouse settings can be configured on the connection level starting from v3.1.0 / v2.2.1