S3 loggyloo (#511)

* add minio-go dep, update deps

* add minio s3 client

minio has an s3 compatible api and is an open source project and, notably, is
not amazon, so it seems best to use their client (fwiw the aws-sdk-go is a
giant hair ball of things we don't need, too). it was pretty easy and seems
to work, so rolling with it. also, minio is a totally feasible option for fn
installs in prod / for demos / for local.

* adds 's3' package for s3 compatible log storage api, for use with storing
logs from calls and retrieving them.
* removes DELETE /v1/apps/:app/calls/:call/log endpoint
* removes internal log deletion api
* changes the GetLog API to use an io.Reader, which is a backwards step atm
due to the json api for logs, I have another branch lined up to make a plain
text log API and this will be much more efficient (also want to gzip)
* hooked up minio to the test suite and fixed up the test suite
* add how to run minio docs and point fn at it docs

some notes: notably we aren't cleaning up these logs. there is a ticket
already to make a Mr. Clean who wakes up periodically and nukes old stuff, so
am punting any api design around some kind of TTL deletion of logs. there are
a lot of options really for Mr. Clean, we can notably defer to him when apps
are deleted, too, so that app deletion is fast and then Mr. Clean will just
clean them up later (seems like a good option).

have not tested against BMC object store, which has an s3 compatible API. but
in theory it 'just works' (the reason for doing this). in any event, that's
part of the service land to figure out.

closes #481
closes #473

* add log not found error to minio land

api/datastore/internal/datastoreutil/validator.go

@@ -130,10 +130,6 @@ func (v *validator) GetCall(ctx context.Context, appName, callID string) (*model
 	return v.Datastore.GetCall(ctx, appName, callID)
 }
 
-func (v *validator) DeleteLog(ctx context.Context, appName, callID string) error {
-	return v.Datastore.DeleteLog(ctx, appName, callID)
-}
-
 // GetDatabase returns the underlying sqlx database implementation
 func (v *validator) GetDatabase() *sqlx.DB {
 	return v.Datastore.GetDatabase()