Check libcurl return value

[dentiny]

This PR does two things:

• (major) Assert libcurl operations' return value, otherwise it could fail silently
• (major) Move header file inclusion out of duckdb namespace
• (minor) Use atomic variable for httpfs client count, so we could (1) reduce one global variable; (2) slightly better perf

[carlopi]

I think first point is valid, unsure if it should be a D_ASSERT (meaning only there on relassert) or then a throw of some kind (unsure which), so that is visible to all users.

Second split-off PR has already been merged.

Third I guess so.

[dentiny]

I think first point is valid, unsure if it should be a D_ASSERT (meaning only there on relassert) or then a throw of some kind (unsure which), so that is visible to all users.

The reason I use check failure here is: we're simply setting configs for curl handle, and they're not expected to fail.

[dentiny]

Current implementation is actually buggy:

• Under release debug mode, D_ASSERT is translated to assert, which does nothing
• So to me, the best practice is never place any real logic inside of D_ASSERT