Store network_service cache in memory

Store network_service cache in memory

The network cache is not safe for usage by multiple processes at the same
time like when sky tests are running many mojo_shell instances in parallel.
To work around this we store the cache in memory. In the future we should
consider file system partitioning for mojo services.

This CL originally reviewed and landed in the Mojo repo in
https://codereview.chromium.org/679403002/. Re-landing in the Chromium repo
because this code is canonically developed here.

TBR=davemoore@chromium.org

Committed: https://crrev.com/07969efff543ef1dffff2ba6a16dd017252eb689
Cr-Commit-Position: refs/heads/master@{#302411}

[abarth-chromium, 2014-11-03 03:29:51]

The CQ bit was checked by abarth@chromium.org

[commit-bot: I haz the power, 2014-11-03 03:30:18]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/697883003/1

[commit-bot: I haz the power, 2014-11-03 04:09:55]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2014-11-03 04:10:41]

Patchset 1 (id:??) landed as
https://crrev.com/07969efff543ef1dffff2ba6a16dd017252eb689
Cr-Commit-Position: refs/heads/master@{#302411}