Avoid StreamController.broadcast

[nielsenko]

There are a number of reasons to prefer not to use broadcast stream here.

1. You can always turn a normal stream into a broadcast stream with asBroadcast, but it is impossible to go the other way.
2. We are not actually sharing the progress stream (and hence the backing StreamController). Each call to getProgressStream will create a new one anyway, so unless the user explicitly shares the returned stream there is no sharing. Fx. our own tests pass just fine, when converting to a normal stream.
3. The SessionProgressNotificationController owning the StreamController (and hence the Stream) is passed to native and become a GC root, preventing the GC to collect until realm-core releases it. Unlike single-subscriber streams which are constructed for one-time-use, a broadcast stream is useful to pass around a be subscribed multiple times. If a single cancel call is missed, we have leak.
4. A pause on a subscription on a broadcast stream will cause events to be lost while paused. Recall that dart will implicitly pause stream during await for loops. I believe it is important that people opt-in to this.

[blagoev]

These could be an argument to never return a broadcast stream from any API. To me it is more of a user limitation and single listener streams should be exposed only when explicitly required by the API.
Since this was exposed to the user as a Stream we used a broadcast stream to allow multiple listeners by default. This also covers single listener case and does not require addition call to asBoradcast stream.
Also isn't this a breaking change.

Happy to discuss this on the round table. Perhaps we could decide to change it.

[coveralls]

Pull Request Test Coverage Report for Build 5875858867

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 6 unchanged lines in 1 file lost coverage.
• Overall coverage increased (+0.03%) to 89.005%

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Files with Coverage Reduction	New Missed Lines	%
lib/src/session.dart	6	83.56%

Totals
Change from base Build 5867151472:	0.03%
Covered Lines:	3311
Relevant Lines:	3720

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💛 - Coveralls