Fix open() concurrency in goroutines (#62)
In short, vsql (with the default options) when dealing with concurrent
read/write access to single file provides the following protections:
- Fine: Multiple processes open() the same file.
- Fine: Multiple goroutines sharing an open() on the same file.
- Bad: Multiple goroutines open() the same file.
The connection_test.v fell into the last category. It's interesting it
wasn't picked up by local testing or CI (at least on the first run
before it was merged).
The test has now been updated to use the new "mutex" option and
increased from 10 goroutines x 10 records to 10 goroutines x 100
records.
