Parallel Processing, exposing -Credential

[FriedrichWeinmann]

Adding a few operational improvements, without affecting the core logic:

• Adding parallel processing - all servers are now processed in parallel, leading to significant performance improvement when running against multiple servers
• Exposing the Credential parameter at the script level. This allows running the script as a different user than the one currently logged on (e.g. when running it from an admin desktop as an exchange admin)
• Improved pipeline implementation to no longer redefine the functions for each server specified
• Improved pipeline implementation to pick up the FQDN of exchange server objects generated from Get-ExchangeServer as input. This will more fluently accept server objects from a different domain in multi-domain forests.
• Updated splatting notation for maximum PowerShell version backwards compatibility