Reland: Refactor current GAE abstraction library to be free of the SDK*

Reland: Refactor current GAE abstraction library to be free of the SDK*

Also improves the struct serialization implementation over the SDK's:
  * 'non-struct' representation is now map[string][]values, instead of
    []PropertyList. This makes working with PropertyLoadSaver much easier.
  * Fields can implement a DSPropertyConverter interface to transparently be
    serialized to/from some other datastore-compatible type. This makes many
    uses of PropertyLoadSaver unnecessary.
  * Lots of reflection code moved out of structPLS.{Load,Save} into the creation
    of the struct codec (e.g. it happens once instead of on every load/save now).
  * Struct codecs are directly accessible by users, allowing easier
    implementation of PropertyLoadSaver interface.
  * Struct codecs support 'special' tagged fields. This will be used for
    implementing an embedded-key Datastore interface without adding any extra
    reflection code.
  * DSProperty's now have strictly private fields. This ensures that every
    DSProperty instance contains only valid types for Value (since it's not
    possible to construct one with an incompatible Value now), which simplifies
    error checking code.
  * DSProperty does automatic upconversion of close-but-not-exact types (like
    MyString -> string, and int -> int64). This upconversion was previously only
    available for struct serialization.
  * DSProperty's now retain a sortable 'Type' field to avoid unnecessary
    additional reflection when serializing them (such as to memcache).
  * There is now a consistent, sortable binary serialization format for
    DSPropertyMap's, which will be used for the follow-on transparent memcache
    implementation. It's also used internally by the in-memory testing
    implementation of the datastore.

*I alias some error types from the new-SDK, but users of the abstraction no
longer need to import the SDK at all.

Original broke because endpoints needed to be moved to a version which relies
on the new SDK.

R=maruel@chromium.org, vadimsh@chromium.org
BUG=

Committed: https://chromium.googlesource.com/infra/infra/+/9d34bb03c2adec2dec0369712b2011b6659e4e54

[iannucci, 2015-07-14 16:33:26]

The CQ bit was checked by iannucci@chromium.org to run a CQ dry run

[commit-bot: I haz the power, 2015-07-14 16:33:39]

Dry run: CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1240573002/20001

[iannucci, 2015-07-14 16:38:39]

The CQ bit was checked by iannucci@chromium.org to run a CQ dry run

[commit-bot: I haz the power, 2015-07-14 16:38:55]

Dry run: CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1240573002/40001

[iannucci, 2015-07-14 16:39:25]

iannucci@chromium.org changed reviewers:
+ dnj@chromium.org

[iannucci, 2015-07-14 16:39:26]

ptal, ps#1 is the original which g'splod'd the other day. I think it should be
less g'splodey now.

[commit-bot: I haz the power, 2015-07-14 16:41:56]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2015-07-14 16:41:58]

Dry run: This issue passed the CQ dry run.

[dnj, 2015-07-14 16:47:41]

rs-lgtm hoping for less g-splosion.

[iannucci, 2015-07-14 16:48:41]

The CQ bit was checked by iannucci@chromium.org

[commit-bot: I haz the power, 2015-07-14 16:48:55]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1240573002/40001

[commit-bot: I haz the power, 2015-07-14 16:52:22]

Committed patchset #3 (id:40001) as
https://chromium.googlesource.com/infra/infra/+/9d34bb03c2adec2dec0369712b201...