Adds checksum flag to zstd codec (#519)

* expose checksum toggle for zstd

* fixes zstd checksumming

* less fixtures

* write_checksum -> checksum

* adds release notes

* set default clevel to 0

* release

* update fixtures

* fix checksum flag

* add test for checksum

* adds wrapper codecs for the v2 codec pipeline

* docstring

Author: normanrz

docs/release.rst

@@ -1,4 +1,4 @@
 {
     "id": "zstd",
-    "level": 1
+    "level": 0
 }

fixture/zstd/codec.00/config.json

@@ -0,0 +1,5 @@
+{
+    "checksum": true,
+    "id": "zstd",
+    "level": 0
+}