Using persistent connections by default

[Nick-S-2018]

Proposal:

To avoid a problem with HTTP connections failing randomly in our different clients (manticoresoftware/manticoresearch-php#243, manticoresoftware/manticoresearch-go#15), we can follow HTTP 1/1 connection management logic and create persistent connections by default instead of making the client to send the Keep-alive header explicitly.
To close such connections, the client must send the Connection: close header.

As shown in the respective tests, a possible caveat that we face an unlimited number of connections won't be an issue.

Checklist:

^{To be completed by the assignee. Check off tasks that have been completed or are not applicable.}

• Implementation completed
• Tests developed
• Documentation updated
• Documentation reviewed
• OpenAPI YAML updated and issue created to rebuild clients