refactor: background worker logic into functions

This way the logic is easier to understand, essentially:

```
delete_expired_responses(guc_ttl);
consume_request_queue(curl_mhandle);
insert_curl_responses(curl_mhandle);
```

Also cleanups tabs and instead uses two spaces for indentation.

src/util.h

@@ -3,6 +3,25 @@
 
 #include <utils/jsonb.h>
 
+#define CURL_EZ_SETOPT(hdl, opt, prm) \
+  do { \
+      if (curl_easy_setopt(hdl, opt, prm) != CURLE_OK) \
+        ereport(ERROR, errmsg("Could not curl_easy_setopt(%s)", #opt)); \
+  } while (0)
+
+#define CURL_EZ_GETINFO(hdl, opt, prm) \
+  do { \
+      if (curl_easy_getinfo(hdl, opt, prm) != CURLE_OK) \
+        ereport(ERROR, errmsg("Could not curl_easy_getinfo(%s)", #opt)); \
+  } while (0)
+
+#define EREPORT_NULL_ATTR(tupIsNull, attr) \
+  do { \
+    if (tupIsNull) \
+      ereport(ERROR, errmsg("%s cannot be null", #attr)); \
+  } while (0)
+
+
 extern struct curl_slist *pg_text_array_to_slist(ArrayType *array,
                                                  struct curl_slist *headers);
 extern void parseHeaders(char *contents, JsonbParseState *headers);