Implement live read replication

This commit adds an http server and client for streaming snapshots
and WAL pages from an upstream Litestream primary to a read-only
replica.

integration/testdata/replicate/http/litestream.0.yml

@@ -0,0 +1,5 @@
+addr: :10001
+
+dbs:
+  - path: $LITESTREAM_TEMPDIR/0/db
+    max-checkpoint-page-count: 20

integration/testdata/replicate/http/litestream.1.yml

@@ -0,0 +1,5 @@
+dbs:
+  - path: $LITESTREAM_TEMPDIR/1/db
+    upstream:
+      url: "$LITESTREAM_UPSTREAM_URL"
+      path: "$LITESTREAM_TEMPDIR/0/db"