The API is no longer fallible, and the Exists::Implicit case should be
matched over instead.

* Fix a number of deprecation warnings.
* Fix linker errors related to Go <=> Rust sqlite usage.

The most interesting bit is the mechanism for dequeing / updating builds
on behalf of a forthcoming builder daemon service.

It uses transaction-scoped postgres advisory locks to offer a build
queue which can be processed in parallel, and which is tolerant to
builder failures.

Issue #400

The builder daemon periodically polls the database for a build to run.
On finding one, it builds it in a local temporary directory using
`flowctl api build`, and then tests it using `flowctl temp-data-plane`
and `flowctl api test`.

This commit conceptualizes each of these activities as "tasks", where
log lines of each task are directly streamed into the build database as
they occur. The build DB can then be its own source-of-truth for all of
its outputs during the build process. We also preserve structured log
information from the build process.

Looking forward, activations and other background tasks will produce
similar types of logs, and we can use the same mechanism to capture /
persist / query them.

For now, the build daemon is directly integrated into the control plane
server. We'll probably want to separate these in the future.

Issue #400

Also add an index on builds.account_id.

* Use new url_for pattern
* State::Done => State::Success
* Terminology: "task" => "job" for background jobs of the control plane.