When executing CALL in PL/pgSQL, we need to set a snapshot before
invoking the to-be-called procedure.  Otherwise, the to-be-called
procedure might end up running without a snapshot.  For LANGUAGE SQL
procedures, this would result in an assertion failure.  (For most other
languages, this is usually not a problem, because those use SPI and SPI
sets snapshots in most cases.)  Setting the snapshot restores the
behavior of how CALL worked when it was handled as a generic SQL
statement in PL/pgSQL (exec_stmt_execsql()).

This change revealed another problem:  In SPI_commit(), we popped the
active snapshot before committing the transaction, to avoid "snapshot %p
still active" errors.  However, there is no particular reason why only
at most one snapshot should be on the stack.  So change this to pop all
active snapshots instead of only one.