Add code-of-conduct.md

Refer to kubernetes/community as authoritative source for code of conduct

Adds a health check endpoint, at the root address.

The health check endpoint is reachable even when basic HTTP authentication
is enabled.

The health check confirms that the backing store is responding, returning an error
response if it is not, else a successful response.

Add X-Request-Id header

New file pkg/chartmuseum/logging.go created to handle all logging

New Logger methods added (Debugc, Infoc, Warnc, Errorc) that take a
gin context as an argument, which will prepend log messages with the
request count, and add request id (reqID) to key-values

Only test storages that are configured in the env

One may not have access to all cloud storages at a time. Allow the
testing framework to run even if it is missing a storage.

In the acceptance test and in the testcloud make targets, Storage tests
will only run for the corresponding defined storage variable i.e.:

Amazon tests will run only if TEST_STORAGE_AMAZON_BUCKET is defined and
Google tests will run only if TEST_STORAGE_GOOGLE_BUCKET is defined.

The build machine should catch any test outliers, as it defines
correctly all the cloud test environment variables.

Fix index response times increasing with many uploads

Index regeneration before this PR was causing a concurrency pile-up when
many requests were coming in at a fast pace.  This was causing the time
to return /index to always increase. The behavior was observed on a high
latency connection to Google Cloud Storage, but would also occur with
other network stores.

With this PR, the regeneration time is constant and thus the serving of
index.yaml is also constant.

The implementation takes a general approach to first make initial
network requests (to satisfy the object list from storage) and to pile
up subsequent requests while the initial one is being completed. The
same algorithm is used to update the in-memory cached index.  Note that
these requests need not be protected by a lock because they are only
ever executing on their own. See server.getChartList() for the
implementaion of this idea.

A refactor was needed to separate the fetch from the diff calculation to
allow separate calling of the network heavy operations. While doing so,
we also removed redundant calls to storage file list update.

Also made small low-hanging fruit style optimisations to index
manipulations.

Added request ID to all requests for better debugging. This will be
visible with the --debug flag. This was indispensable to diagnose
complex concurrent processing.

To test the before and after state, we have added the use of the
locusti.io loadtesting engine. A simple README in the loadtesting/
directory shows how to install locust (with pipenv) and loadtest
chartmuseum. This will prove useful in the future.

Fixes #18

Update project paths to kubernetes-helm/chartmuseum

Since ChartMuseum has moved to the kubernetes-helm incubator, it can
now be found in GitHub under the kubernetes-helm project. In keeping
with this, update the project paths from chartmuseum/chartmuseum to
kubernetes-helm/chartmusuem.

Fix concurrency issue in presence of download errors

Use channel based flow control to fix concurrency when a download error occurs. Read chartVersion from channel to add to the index as they arrive.

Add context management to cancel any pending downloading goroutines.

Add function to limit request counter cardinality