Allow multi-map async queries to be unbuffered

[mburbea]

See #596 for more detail. Originally, the code would dispose the
dataReader that it opened before enumeration would begin. Now when you
dispose the enumerator, it will also close the command, which is line
with other behavior.
Buffered behavior remains consistent. Has no impact on sync version.

[mburbea]

@mgravell , I'll fix the merge conflict but is there any object to merging this?

[mgravell]

Needs a test/usage-illustration, but other than that: looks good

[NickCraver]

Any update on this?

[NickCraver]

@mburbea Any update on example usage/test for this?

[mburbea]

@NickCraver, hey sorry it's been a while since I looked at this issue I'm not currently working on a project that uses the dapper library (that was back in september) so I have not had time to look further into this. If you rather I close the issue. I can do that.

[Karillion]

Would be great to see this fixed in the current version of Dapper rather than 2,0. That seems to be a way off still.

[davelovesoftware]

Is there a reason this can't get merged (or otherwise fixed) until V2? I'm working on something at the moment that uses only async calls and could really benefit from disabling buffering in several places.