Serialize connection access

[czeidler]

I wasn't aware of your repo and did my own version of kysely-pglite. I had a problem where the connections where not serialized, e.g. concurrent tasks mixed up queries and transactions. I solve it by serializing the access to the connections. Feel free to copy:

https://github.com/czeidler/kysely-pglite/blob/82cdb067fa5668084ef7f290d998f9dae07b49f2/src/dialect.ts#L53