Set max connections for plan cmd

[bplunkett-stripe]

Description

Now that we fetch schemas in parallel, we should set a maximum number of connections on the connection pool for planning.

Motivation

Follow best practices

Testing

 go run ./cmd/pg-schema-diff apply  --dsn "host=localhost user=postgres password=postgres database=somedb" --schema-dir ~/stripe/temp/examplesql
################################## Review plan ##################################
1. ALTER TABLE "public"."foobar" DROP CONSTRAINT "some_other_constraint";
        -- Statement Timeout: 3s

✔ Yes
############################# Executing statement 1 #############################
ALTER TABLE "public"."foobar" DROP CONSTRAINT "some_other_constraint";
        -- Statement Timeout: 3s

Finished executing statement. Duration: 6.794458ms
################################### Complete ###################################
Schema applied successfully