Use the mutex in exvar.Set since map access is not atomic.
Use the mutex in exvar.Set since map access is not atomic.

Imagine your var has a value of zero. If you have a goroutine calling Set(5),
and another calling Increment(+1), then you only want one of these outcomes:
  - Set completes first, and then Increment occurs => 6
  - Increment completes first, and then Set occurs => 5

However, you could get a sequence:
  - read (for Increment) 0
  - set (for Set) 5
  - write (for Increment) 1
This results in a value of 1, which is undesirable.

Kudos to dnadasi for catching this.

R=r
APPROVED=r
DELTA=3  (3 added, 0 deleted, 0 changed)
OCL=27625
CL=27625

tianon/debian-golang,gnanderson/go,dsriram/golang,tv42/old-go,gnanderson/go,gnanderson/go,dsriram/golang,dsriram/golang,bdwalton/debian-golang,bdwalton/debian-golang,nightlyone/golang,ssrl/go,wesolows/golang,wesolows/golang-old,pombredanne/go-deleteme,bdwalton/debian-golang,h8liu/golang,wesolows/golang,tv42/old-go,gnanderson/go,apparentlymart/golang,dsriram/golang,h8liu/golang,ssrl/go,ssrl/go,dsriram/golang,h8liu/golang,tianon/debian-golang,apparentlymart/golang,wesolows/golang,ssrl/go,pombredanne/go-deleteme,nightlyone/golang,h8liu/golang,wesolows/golang,apparentlymart/golang,nightlyone/golang,tv42/old-go,h8liu/golang,ssrl/go,h8liu/golang,nightlyone/golang,firebitsbr/golang-old,tianon/debian-golang,gnanderson/go,tv42/old-go,gnanderson/go,gnanderson/go,tv42/old-go,ssrl/go,wesolows/golang,pombredanne/go-deleteme,firebitsbr/golang-old,nightlyone/golang,firebitsbr/golang-old,tv42/old-go,wesolows/golang,pombredanne/go-deleteme,firebitsbr/golang-old,firebitsbr/golang-old,dsriram/golang,tianon/debian-golang,tianon/debian-golang,tianon/debian-golang,dsriram/golang,bdwalton/debian-golang,wesolows/golang-old,firebitsbr/golang-old,wesolows/golang,firebitsbr/golang-old,bdwalton/debian-golang,bdwalton/debian-golang,bdwalton/debian-golang,wesolows/golang-old,pombredanne/go-deleteme,pombredanne/go-deleteme,tv42/old-go,apparentlymart/golang,tianon/debian-golang,nightlyone/golang,apparentlymart/golang,wesolows/golang-old,bdwalton/debian-golang,nightlyone/golang,tianon/debian-golang,pombredanne/go-deleteme,wesolows/golang-old,wesolows/golang-old,apparentlymart/golang,apparentlymart/golang,h8liu/golang,firebitsbr/golang-old,wesolows/golang-old,ssrl/go