Add isOpen and closed

[simolus3]

The two getters allow checking the current state of a postgres session, as well as getting notified when it closes. This combination mirrors the API of asynchronous primitives (like StreamConsumer or StreamChannel).

I think this should close the remaining concern of #119, but as @osaxma suggested we might want to wait until we have a clearer understanding of the requirements for this.

[codecov-commenter]

Codecov Report

Attention: 6 lines in your changes are missing coverage. Please review.

Comparison is base (1cf35f1) 86.75% compared to head (65e3997) 86.76%.
Report is 4 commits behind head on master.

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #222      +/-   ##
==========================================
+ Coverage   86.75%   86.76%   +0.01%     
==========================================
  Files          32       32              
  Lines        2779     2804      +25     
==========================================
+ Hits         2411     2433      +22     
- Misses        368      371       +3     

Files	Coverage Δ
lib/postgres.dart	57.14% <ø> (ø)
lib/src/auth/clear_text_authenticator.dart	100.00% <ø> (ø)
lib/src/auth/md5_authenticator.dart	0.00% <ø> (ø)
lib/src/auth/sasl_authenticator.dart	88.57% <ø> (ø)
lib/src/exceptions.dart	91.48% <ø> (ø)
lib/src/message_window.dart	94.54% <ø> (ø)
lib/src/messages/client_messages.dart	83.21% <ø> (ø)
lib/src/messages/logical_replication_messages.dart	69.59% <ø> (ø)
lib/src/messages/server_messages.dart	80.48% <ø> (ø)
lib/src/messages/shared_messages.dart	30.00% <ø> (ø)
... and 5 more

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[isoos]

I like this. The only thing I would consider is to use isClosed (vs isOpen) alongside with closed and maybe eventually close() (if we ever have non-callback based Sessions...). But this is good as-is, merging for now, and if you feel different about the open/close part, we can change it.