[Orchestrator] Increase the connection limit

[loic-sharma]

The Orchestrator used the default connection limit of 2 per server. The Orchestrator processes messages in parallel, each of which may be downloading/uploading large files.

Part of NuGet/NuGetGallery#6624

Performance Test

I ran performance tests where I enqueued the same ~1,500 packages for validation. I then found how long it took the Orchestrator to download packages before and after this change:

Test	Average	90%	99%	99.9%
Before	9.64s	19.87s	32.58s	37.05s
After	6.59s	13.90s	24.96s	33.14s

traces
| where timestamp > todatetime('2018-11-21T23:55:00.000')
| where timestamp < todatetime('2018-11-22T03:50:00.000')
| where cloud_RoleName == "NuGet.Services.Validation.Orchestrator"
| where message contains "Downloaded" and message contains "bytes in" and message contains "seconds for request"
| extend DownloadSeconds = todouble(customDimensions.DownloadElapsedTime) 
| extend AfterIncreasingConnectionLimit = iff(timestamp > todatetime('2018-11-22T02:10:00.000'), 1, 0)
| summarize count(), avg(DownloadSeconds), percentiles(DownloadSeconds, 90, 99, 99.9) by AfterIncreasingConnectionLimit
| order by AfterIncreasingConnectionLimit  asc