Update isConnected check to use PQStatus

[DunnCoding]

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[codecov-io]

Codecov Report

Merging #49 into master will not change coverage.
The diff coverage is 0%.

@@           Coverage Diff           @@
##           master      #49   +/-   ##
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  Coverage   74.21%   74.21%           
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  Files           3        3           
  Lines         512      512           
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  Hits          380      380           
  Misses        132      132

Flag	Coverage Δ
#SwiftKueryPostgreSQL	74.21% <0%> (ø)	⬆️

Impacted Files	Coverage Δ
...es/SwiftKueryPostgreSQL/PostgreSQLConnection.swift	63.81% <0%> (ø)	⬆️

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[djones6]

This fixes the check so that we are asking libpq for the status, and does eventually give the right answer, and is an improvement which is worth making.

Unfortunately, it turns out that libpq does not do anything more than look up a flag when you call this: if the connection hasn't been used since it died, this will still answer CONNECTION_OK. The consequence is that the user will get a connection which claims to be OK, but will fail when they try to use it. After that point, it will correctly answer that it is dead.

One way that we might be able to make Postgres connections behave nicely would be something like:

• attempt execution of a query
• if the attempt fails, check PQstatus:
  • if the connection wasn't dead, just return the error as normal
  • if it is dead, attempt to replace our libpq connection with a new one. If this succeeds, execute the query again, otherwise, return the original error.

There are various issues with this which need more thought, and it's way out of scope for this PR.