BUG/MEDIUM: lua: the function txn_done() from sample fetches can crash

BUG/MEDIUM: lua: the function txn_done() from sample fetches can crash

The function txn_done() ends a transaction. It does not make
sense to call this function from a lua sample-fetch wrapper,
because the role of a sample-fetch is not to terminate a
transaction.

This patch modify the role of the fucntion txn_done() if it
is called from a sample-fetch wrapper, now it just ends the
execution of the Lua code like the done() function.

Must be backported in 1.6