[SCHEMA] Rename account.default_calendar_id foreign key constraint.

Summary:
This is an attempt to work around this bug:
http://bugs.mysql.com/bug.php?id=70171
in MySQL versions 5.6.12 and 5.6.13, which was causing migration 083 to fail
when run against RDS.

Test Plan:
Run migrations both on a local VM previously synced to head, and on
gunks.

Reviewers: charles

Reviewed By: charles

Differential Revision: https://review.inboxapp.com/D388
[SCHEMA] Simplify transaction log construction.

Summary:
This commit implements a leaner, meaner transaction log. Instead of storing an
ORM-derived delta, only store a snapshot of the API representation of the
versioned object. This reduces the volume of data we store, makes constructing
the transaction faster, and removes some complexity left over from when we were
trying to infer syncback actions from the transaction log. It's way less
generic, but better suited to our actual application.

Includes some simplifications to the Lens model -- just removing parts that
would be broken by this change, and were part of webhooks but are unlikely to
be repurposed into ping.

Test Plan: Need to add some unit tests before landing.

Reviewers: spang

Differential Revision: https://review.inboxapp.com/D385
Use an S3 connection pool for blob saving.

It turns out that initializing a new S3 connection for every object you save to
S3 is really expensive. Instead, maintain a pool of worker greenlets, each of
which maintains a persistent S3 connection. (Note that an S3 connection cannot
be shared across greenlets.)