Implement the main flow for the Exporting Engine (#7149)

* Add top level tests

* Add a skeleton for preparing buffers

* Initialize graphite instance

* Prepare buffers for all instances

* Add Grafite collected value formatter

* Add support for exporting.conf read and parsing

* - Use new exporting_config instead of netdata_config

* Implement Grafite worker

* Disable exporting engine compilation if libuv is not available

* Add mutex locks

- Configure connectors as connector_<type> in sections of exporting.conf

- Change exporting_select_type to check for connector_ fields

* - Override exporting_config structure if there no exporting.conf so that
  look ups don't fail and we maintain backwards compatibility

* Separate fixtures in unit tests

* Test exporting_discard_responce

* Test response receiving

* Test buffer sending

* Test simple connector worker

- Instance section has the format connector:instance_name
  e.g graphite:my_graphite_instance

- Connectors with : in their name e.g graphite:plaintext are reserved
  So graphite:plaintext is not accepted because it would activate an
  instance with name "plaintext"
  It should be graphite:plaintext:instance_name

* - Enable the add_connector_instance to cleanup the internal structure
  by passing NULL,not NULL arguments

* Implement configurable update interval

- Add additional check to verify instance uniqueness across connectors

* Add host and chart filters

* Add the value calculation over a database series

* Add the calculated over stored data graphite connector

* Add tests for graphite connector

* Add JSON connector

* Add tests for JSON formatting functions

* Add OpenTSDB connector

* Add tests for the OpenTSDB connector

* Add temporaty notes to the documentation

1 files changed, 6 insertions, 1 deletions

diff --git a/backends/backends.h b/backends/backends.h
index 9da04ccee4..b259141555 100644
--- a/backends/backends.h
+++ b/backends/backends.h
@@ -23,9 +23,13 @@ typedef enum backend_types {
     BACKEND_TYPE_JSON,                      // Stores the data using JSON.
     BACKEND_TYPE_PROMETEUS,                 // The user selected to use Prometheus backend
     BACKEND_TYPE_KINESIS,                   // Send message to AWS Kinesis
-    BACKEND_TYPE_MONGODB                    // Send data to MongoDB collection
+    BACKEND_TYPE_MONGODB,                   // Send data to MongoDB collection
+    BACKEND_TYPE_NUM                        // Number of backend types
 } BACKEND_TYPE;
 
+#ifdef ENABLE_EXPORTING
+#include "exporting/exporting_engine.h"
+#endif
 
 typedef int (**backend_response_checker_t)(BUFFER *);
 typedef int (**backend_request_formatter_t)(BUFFER *, const char *, RRDHOST *, const char *, RRDSET *, RRDDIM *, time_t, time_t, BACKEND_OPTIONS);
@@ -38,6 +42,7 @@ extern BACKEND_OPTIONS global_backend_options;
 extern const char *global_backend_prefix;
 
 extern void *backends_main(void *ptr);
+BACKEND_TYPE backend_select_type(const char *type);
 
 extern BACKEND_OPTIONS backend_parse_data_source(const char *source, BACKEND_OPTIONS backend_options);