avoid read kv for each new session's first select @@variable

[lysu]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. What did you do?
   If possible, provide a recipe for reproducing the error.

when user uses jdbc or spring, it will auto send many select variable statement like:

select @@tx_ready_only;
select @@net_buffer_length;
select @@max_allowed_packet;
select @@character_set_sever
....and so on

those statements will be frequent send, it's better preload their global variable value into TiDB memory.

We have some other variable already use this mechanism, but some other variable is missed(like above)

tidb/session/session.go

Line 1614 in b0549b7

var builtinGlobalVariable = []string{

3. What did you see instead?

frequent used variable's select @@variable, should read session variable or load default value from TiDB local cache, instead of load from kv everytime

4. What version of TiDB are you using (tidb-server -V or run select tidb_version(); on TiDB)?

3.0.1.beta1