query cancel issues in contrib/dblink

Hi,

contrib/dblink seems to have no treatments for query cancels.
It causes the following issues:

(1) Users need to wait for completion of remote query.
Requests for query cancel won't be delivered to remote servers.

(2) PGresult objects will be memory leak. The result is not released
when query is cancelled; it is released only when dblink function
is called max_calls times.

They are long standing issues (not only in 8.4),
but I hope we will fix them to make dblink more robust.

For (1), asynchronous libpq functions should be used instead of blocking
ones, and wait for the remote query using a loop with CHECK_FOR_INTERRUPTS().
For (2), we might need to store PGresult not only in funcctx->user_fctx
but also in a global list, and free all results in XactCallback if remain.

Comments welcome.

Regards,
---
ITAGAKI Takahiro
NTT Open Source Software Center