runtime: make map reads multithreaded safe.

Doing grow work on reads is not multithreaded safe.
Changed code to do grow work only on inserts & deletes.

This is a short-term fix, eventually we'll want to do
grow work in parallel to recover the space of the old
table.

Fixes issue 5120.

R=bradfitz, khr
CC=golang-dev
https://codereview.appspot.com/8242043