go-mode.el: fix syntax highlighting of backticks

go-mode.el: fix syntax highlighting of backticks

Instead of syntax-tables, an extended go-mode-cs is used for
from a font-lock callback.

Cache invalidation must happen in a before-change-function
because font-lock runs in an after-change-function, potentially
before the cache invalidation takes place.

Performance is reasonable, even with src/pkg/html/entity.go
and test/fixedbugs/bug257.go.

Fixes #2330.

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/5529045