go_ora.AddSessionParam(db, k, v) put values to global driver hashmap

[rheilek]

If the go program connect to more than one database/schema and using go_ora.AddSessionParam the last call to AddSessionParam overwrites other values.

for example:

• db1 := db.Open("db1.example.com")
• go_ora.AddSessionParam(db1, "CURRENT_SCHEMA", "WS")
• db2 := db.Open("db2.example.com")
• AddSessionParam(db2, "CURRENT_SCHEMA", "TEST")
• db1.Exec(..) -> ORA-01435: user does not exist - because TEST was used at initConnection

The cause of this is the global hashmap in OracleDriver used in AddSessionParam.

func AddSessionParam(db *sql.DB, key, value string) error {
	...
	if drv, ok := db.Driver().(*OracleDriver); ok {
		drv.mu.Lock()
		defer drv.mu.Unlock()
		drv.sessionParam[key] = value # issue
		...

func (driver *OracleDriver) init(conn *Connection) error {
	...
	for key, value := range driver.sessionParam {
		_, err = conn.Exec(fmt.Sprintf("alter session set %s=%s", key, value))
		...

Because of implementing database/sql maybe the ConnectString might be the right place to do something like this. If you agree i can create a pull request. Otherwise AddSessionParam should be removed from api and every developer must implement his own initialization. Or do you see another approach?