Add Connection{Idle}Timeouts to ManagedHandler (dotnet/corefx#26839)

Add Connection{Idle}Timeouts to ManagedHandler (dotnet/corefx#26839)

* Add Connection{Idle}Timeouts to ManagedHandler

We can agree on the final name(s) when going through API reviews.  For now, I chose what I think are easy to understand names:
- ConnectionTimeout: how long a connection is allowed to be reused before it's no longer usable, defaults to infinite
- ConnectionIdleTimeout: how long a connection can sit idle in the pool before it's no longer usable, defaults to 2 minutes (that value was hardcoded before this change)

* Address PR feedback

Commit migrated from https://github.com/dotnet/corefx/commit/1a5cdbe1af7a5befe16bb4581f29f0af64f4a101