[2.8] Upgrade to HC4.3.2

HC4.3.x introduced a lot of changes:
* DefaultHttpClient is deprecated and by default unused (unless explicitly created)
* Newly created HttpClient instances (implemented by private inner class) are immutable
* new pooling connection manager, new socket factories, a lot of new stuff
* enforcing immutability, hence, a lot of XXX.getParams() is now deprecated, and client.getParams() now throws IllegalOperationEx.
* only HttpContext may be use to pass over contextual information into processing chain

This pull basically reworks the RRS and internal use of HttpClient in Nx, except for:
* use of HC in Nexus Client (still pending, due to Jersey)
* use of HC in Nexus ITs (NexusRestClient uses Restlet client that implicitly uses HC "old way")

CI
https://bamboo.zion.sonatype.com/browse/NX-OSSF29