[TT-1969] - Gateway should synchronise only changed objects from the dashboard or MDCB

[buger]

Is your feature request related to a problem? Please describe.
At the moment, if you have lot of policies or APIs (thousands), each gateway reload takes a lot of time (up to a minute). And any policy or api change/update can trigger such reload.

In some of the flows, it is required dynamically update policies, and such delays could be unacceptable.

Describe the solution you'd like
It will be great if we will be able to just sync only the APIs/Policies, which actually got changed.
Dashboard/MDCB can define new APIs, which accept gateway current state like this:

{
  "policies": {"<policy-id>": "<last-updated-date>"},
  "apis": {"<api-id>": "<last-updated-date>"} 
}

And in response it should get response like:

{
   "updated": {"<policy-id>": <policy-object>},
   "deleted": ["<policy-id>"],
   "created": {"<policy-id>": <policy-object>},
}

The same response should be for the APIs.

We can make it backward compatible, and first check new "POST" endpoint, and after old "GET" endpoint.

[matiasinsaurralde]

Update: I've started to work on this, first on the gateway side. For now I've isolated all the functionality into a function inside DoReload so that's called instead of syncPolicies and syncAPISpecs (from the above description I've assumed that we'll unify both calls).
An interesting finding is that we don't currently have a last_updated field in the API definition. Will need to handle this on the dashboard side. I will focus on the dashboard side today.