add subscription http endpoints

[mikemrm]

Description

Adds http endpoints to list or get subscribers. This allows for tooling to detect whether a server is currently connected to hegel to determine how it should proceed.

Example Output

$ curl -s localhost:50061/subscriptions | jq '.'
{
  "abcdefab-cdef-abcd-efab-abcdefabcdef": {
    "id": "abcdefab-cdef-abcd-efab-abcdefabcdef",
    "ip": "127.0.0.1",
    "init_duration": 2000004,
    "started_at": "2020-08-11T20:55:04.56156941Z"
  }
}

$ curl -s localhost:50061/subscriptions/abcdefab-cdef-abcd-efab-abcdefabcdef | jq '.'
{
  "id": "abcdefab-cdef-abcd-efab-abcdefabcdef",
  "ip": "127.0.0.1",
  "init_duration": 2000004,
  "started_at": "2020-08-11T20:55:04.56156941Z"
}

$ curl -s localhost:50061/subscriptions/abcdefab-cdef-abcd-efab-abcdefabcdee | jq '.'
{
  "error": {
    "comment": "item not found",
    "error": "abcdefab-cdef-abcd-efab-abcdefabcdee not found"
  }
}

[mikemrm]

@kdeng3849 tagging you on here for visibility. I've had to make quite a few changes in order to make hegel backwards compatible with osie/osie preinstall. Issues were primarily with metadata instance, and storage attributes.

[mmlb]

This commit message is actually missing the most important part, that the suffix is very important to tools we run that uses this data, so needs to stay (sgdisk)

[mmlb]

Don't need the ok check I think since for the == to be true then ok will also be true. If ok is false, there's no way cSub.updateChan == sub.updateChan because cSub.updateChan will be nil and sub.updateChan is never nil at this point.

[mmlb]

Also deserves some sort of comment that this is a guard against deleting a sub that has already been deleted because of a reconnection attempt or similar.

[mikemrm]

cSub could be null if the id doesn't already exist. But will add a comment

[mmlb]

right, but sub isn't null so they don't == so you wont delete

[mikemrm]

on a normal disconnect, cSub and sub could be equal. As another connection would not have come in and overwrite the s.subscriptions[id]

[mmlb]

This commit message is actually missing the most important part, that the suffix is very important to tools we run that uses this data, so needs to stay (sgdisk)

@mikemrm ^

[mikemrm]

Sorry, missed that. I've added more details about the sgdisk in the comment.