Introduce Settings.HTTP_MAX_CONNECTIONS to regulate maximum connections to specific endpoints for capability requests.

Previously, CapsBase.SetupRequest() hardcoded this to 32 and DownloadManager.ParallelDownloads was setting this with a default value of 8.
This meant that the ConnectionLimit would oscillate rapidly between these two figures as requests were made.
On Mono 3.2.8 and quite possibly other Mono, this appears to increase the chance that the VM will crash under heavy request load.
This commit makes both SetupRequest() calls use the static Settings.HTTP_MAX_CONNECTIONS - other static settings already exist, and so also makes this configurable.
DownloadManager.ParallelDownloads remains a separate setting since it also governs max parallel downloads through a separate coded mechanism.