fix(server): Thread request context to all relevant places

Previously background contexts where created where necessary (e.g. in
GCS interactions). Should I begin to use request timeouts or other
context-dependent things in the future, it's useful to have the actual
HTTP request context around.

This threads the request context through the application to all places
that need it.

tools/nixery/server/storage/storage.go

@@ -4,10 +4,13 @@
 package storage
 
 import (
+	"context"
 	"io"
 	"net/http"
 )
 
+type Persister = func(io.Writer) (string, int64, error)
+
 type Backend interface {
 	// Name returns the name of the storage backend, for use in
 	// log messages and such.
@@ -19,14 +22,14 @@ type Backend interface {
 	// It needs to return the SHA256 hash of the data written as
 	// well as the total number of bytes, as those are required
 	// for the image manifest.
-	Persist(string, func(io.Writer) (string, int64, error)) (string, int64, error)
+	Persist(context.Context, string, Persister) (string, int64, error)
 
 	// Fetch retrieves data from the storage backend.
-	Fetch(path string) (io.ReadCloser, error)
+	Fetch(ctx context.Context, path string) (io.ReadCloser, error)
 
 	// Move renames a path inside the storage backend. This is
 	// used for staging uploads while calculating their hashes.
-	Move(old, new string) error
+	Move(ctx context.Context, old, new string) error
 
 	// Serve provides a handler function to serve HTTP requests
 	// for layers in the storage backend.