Keep alive agent is implementation

[koltyakov]

This PR is based on ideas suggested by @airportyh PR, but with minor changes and HTTPS/HTTP aware for enabling agent only for HTTPS.

Integration tests showed up to 55% increase in speed in On-Prem SSL environment and up to 30% in SPO.

The figures are relative but include some runs with the following proxy operations:

• should get web's title
• should work with shorthand URIs
• should get lists on the web
• should create a new list
• should create a list item
• should update a list item
• should delete a list item
• should fetch minimalmetadata
• should fetch nometadata
• should create list items in a batch
• should create list items in a batch
• should update list items in a batch
• should delete list items in a batch
• should add item's attachment
• should delete a list
• should create a document library
• should add a document
• should delete a document library
• should get web's title with SOAP
• should get web's title with CSOM