feat(pgsrv): Implement start of postgres extended query protocol

[justinrubek]

Implements many of the code paths needed for the protocol. There are a lot of things that need worked on. This implements some functionality that can be used against the benchbase tests, but the implementation of many messages is limited:

• Parse messages create a prepared statement on the session
  • this stores the sql string but doesn't parse it for placeholders
• Bind messages create a portal on the session
  • the sql statement is planned but no placeholder replacement is performed

[justinrubek]

The extended protocol needs the following messages handled

• Parse
  • Parse statement and find locations to replace for parameters
  • Store inside session for later retrieval
• Bind
  • plan query
  • Store inside session for later retrieval
• Execute
  • Run query from portal
• Describe
  • portal
    • Ddl
    • Write
    • Query
    • Transaction
  • statement
• Sync
• Terminate

[justinrubek]

I've turned #101 into a tracking issue for the bigger picture of getting the extended query protocol in. I think this is may be better done in a few parts

[scsmithr]

Very nice.

One thing we'll probably want to add once complete is some sqllogictests verifying behavior for the extended protocol.

[RustomMS]

Neat snippet to use when setting up a vec with known size. Not sure how exactly this compares though with respect to performance.

And not a all necessary, just something I thought I would share.

Suggested change

	let mut param_types = Vec::with_capacity(num_params);
	for _ in 0..num_params {
	param_types.push(buf.get_i32());
	}
	let param_types = (0..num_params).map(|| buf.get_i32()).collect();