Add MemoryStore Write and ReadFrom benchmarks.
Add MemoryStore Write and ReadFrom benchmarks.

  $ go test -run=NONE -bench="MemoryStoreWrite|MemoryStoreReadFrom"
  PASS
  BenchmarkMemoryStoreWrite1000   	     300	   5762982 ns/op
  BenchmarkMemoryStoreReadFrom1000	     100	  10152279 ns/op
  ok  	sourcegraph.com/sourcegraph/appdash	3.366s

According to pprof most of the time spent is in gob -- which seems
accurate.

joeshaw/appdash,joeshaw/appdash,joeshaw/appdash,bg451/appdash,bg451/appdash,bg451/appdash