Add provider HasPending method

[mfridman]

Fix #640

This PR adds a new provider method to check for pending migrations.

func (p *Provider) HasPending(context.Context) (bool, error) {}

This functionality is especially useful for cases like @roblaszczak described in #507 (comment) (I'll try to summarize this when writing up the documentation for this).

Important

This underlying implementation does not respect the SessionLocker and can be used to check for pending migrations without blocking or being blocked by other operations.

I've also added this before running .Up(), .UpTo, .UpByOne methods, which means those methods don't have to acquire the session lock (if enabled) until it's truly required to begin an operation.

This is quite necessary in k8s-type environments where newer pods have long-running migrations, and older pods being bounced/rebooted/recovering from a crash/etc. are blocked by the SessionLocker and the entire application is down waiting on the long-running migration from one of the newer pods to complete.

There's an edge case that needs to be accounted for. Since HasPending doesn't use a SessionLocker, we need to make the initial table version creation retriable since there can be multiple instances trying to create it in parallel. Worst case instance 1 creates the table and instance 2 fails on its first try, then on the second try it succeeds. Retry 3 times every 1 second.

Debug notes

seq 100 | xargs -I{} -P 4 -n 1 sh -c 'go test -timeout 30s -run ^TestPostgresHasPending$ ./internal/testing/integration -race -count=1 >> log.out'