Handle failure of ConnectionPool connection generator

[djones6]

Follows on from #128 and #103.

When the ConnectionPool is first created but the database is unreachable, it can fail to generate connections (if the generator has been written in such a way that only 'good' connections are returned).

In this scenario, the pool's current capacity (the initial number of connections we try to generate) is out of sync with the actual size of the pool (represented by pool.count), which is normally only the case when connections have been vended from the pool and not yet returned. We cannot recover from this situation, as we will not attempt to grow the pool, as its capacity is (apparently) not zero.

This fix ensures that the initial capacity and pool.count are in sync immediately after connection generation. We will attempt to generate a new connection each time we are asked to vend one, and this will continue to fail (and the capacity remain zero) while the database is unreachable. At some future time, if the database were to become reachable again, we will be at zero capacity and begin to grow the pool, as per #128.

Testing

I've verified that this resolves the ability to grow the pool after initial failure using a local project with a Postgres database and a connection generator function that only produces connections if they successfully connect().

[codecov-io]

Codecov Report

Merging #137 into master will decrease coverage by 0.14%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master     #137      +/-   ##
==========================================
- Coverage   84.47%   84.33%   -0.15%     
==========================================
  Files          43       43              
  Lines        3607     3613       +6     
==========================================
  Hits         3047     3047              
- Misses        560      566       +6

Flag	Coverage Δ
#SwiftKuery	84.33% <0%> (-0.15%)	⬇️

Impacted Files	Coverage Δ
Sources/SwiftKuery/ConnectionPool.swift	0% <0%> (ø)	⬆️

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