Rename #generalizedLocalTx to #localTxForReturnType
Add localTxFor as a shorten name of localTxForReturnType
Hide localTxForReturnType from library users in 2.1

 - make localTxForReturnType library private
 - remvoe localTxFor alias (same in 2.2)
 - TxBoundary type class instances for Try or Either won't be provided by default
Merge pull request #303 from scalikejdbc/feature/localTxFor

Rename #generalizedLocalTx to #localTxForReturnType