cmd/geth, mobile: add memsize to pprof server (#16532)

* cmd/geth, mobile: add memsize to pprof server

This is a temporary change, to be reverted before the next release.

* cmd/geth: fix variable name

vendor/github.com/fjl/memsize/doc.go

@@ -0,0 +1,16 @@
+/*
+Package memsize computes the size of your object graph.
+
+So you made a spiffy algorithm and it works really well, but geez it's using
+way too much memory. Where did it all go? memsize to the rescue!
+
+To get started, find a value that references all your objects and scan it.
+This traverses the graph, counting sizes per type.
+
+    sizes := memsize.Scan(myValue)
+    fmt.Println(sizes.Total)
+
+memsize can handle cycles just fine and tracks both private and public struct fields.
+Unfortunately function closures cannot be inspected in any way.
+*/
+package memsize