Add support for modern cancel functions (PQcancel deprecated)

[vog]

PostgresSQL-OCaml currently wraps only the deprecated PQcancel function. The modern cancel functions can be found at:

• https://www.postgresql.org/docs/17/libpq-cancel.html

At the very minimum, for async clients following functions are needed:

• PQcancelCreate
• PQcancelFinish
• PQcancelStart
• PQcancelPoll
• PQcancelSocket

And for sync clients:

• PQcancelCreate
• PQcancelFinish
• PQcancelBlocking

[mmottl]

I'm only maintaining the bindings these days, please feel free to submit a PR.

[vog]

I'd like to give it a try if I find some time!

But one question upfront: These new functions are available starting with PostgreSQL 17, while PostgreSQL 16, 15 and 14 are still officially supported. What is the recommended strategy to deal with potential compatibility issues?

• Shall we wait until PostgreSQL 17 is the oldest officially supported PostgreSQL version? (That might be too extreme.)
• Shall we just implement it, and make libpq >= 17 a strict dependency?
  • If so, how do we then deal with situations of connecting to servers <= 16?
• Alternatively, can we implement it in a way where we dynamically decide whether to call those functions or not, depending e.g. on their availability and/or the libpq version?

The very some questions apply of course to #67.

[mmottl]

It should generally be safe to just update and use version constraints. People still tied to older infrastructure can pin the package to the version they require if necessary. The bindings don't usually evolve so quickly that people would be missing out on new features or bug fixes.

Introducing dynamic decisions on how to connect would seem overly complicated. I'm not sure how sophisticated libpq is when connecting to older servers. I would think it can, but it might fail if it had to use unsupported protocol features.