Add pagination system to http endpoint executions/list

[NicolasMahe]

Added a simple and not optimized pagination system on executions/list HTTP endpoint so the explorer can fetch executions without breaking.

The parameters page, limit and sort=reverse can be passed to the endpoint to return different executions:

curl 'localhost:1317/execution/list?page=12&limit=1&sort=reverse'

The sort is based on the execution's blockHeight

Related to #1679

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Improvements that can be done now or later:

1. Move the pagination and sort system in the module's querier but need to pass as parameters page, limit and sort.
2. Move the pagination and sort system in the module's keeper and use KVStorePrefixIteratorPaginated but need to pass as parameters page, limit and find a way to sort the data without iterate on all of them (index?)

[krhubert]

I hope this is only solution until next sprint.

[krhubert]

We should return error if the pagination boundaries are invalid (so in that case check if page/limit <= 0), otherwise returning empty list.

s/execs = make([]*execution.Execution, 0)/execs = nil/

[NicolasMahe]

the page and limit <= 0 error is managed by rest.ParseHTTPArgs(r):

} else if page <= 0 {
	return tags, page, limit, errors.New("page must greater than 0")
}

} else if limit <= 0 {
	return tags, page, limit, errors.New("limit must greater than 0")
}

---

the problem with returning execs = nil is it's not a list anymore in the json response:

{"height":"757","result":null}

so we will have issue with clients.
i would keep it as it is:

{"height":"773","result":[]}

i replaced the make([]*execution.Execution, 0) by simply []*execution.Execution{}

[antho1404]

Except for one suggestion, lgtm

[NicolasMahe]

@krhubert @antho1404 modif done. please check again