multi: Simplify code per gosimple linter.

This simplifies the code based on the recommendations of the gosimple
lint tool.

Also, it increases the deadline for the linters to run to 10 minutes and
reduces the number of threads that is uses. This is being done because
the Travis environment has become increasingly slower and it also seems
to be hampered by too many threads running concurrently.

txscript/stack.go

@@ -155,7 +155,7 @@ func (s *stack) nipN(idx int32) ([]byte, error) {
 	if idx == 0 {
 		s.stk = s.stk[:sz-1]
 	} else if idx == sz-1 {
-		s1 := make([][]byte, sz-1, sz-1)
+		s1 := make([][]byte, sz-1)
 		copy(s1, s.stk[1:])
 		s.stk = s1
 	} else {