Assorted fixes, mostly for sbcl

There were some problems with recent cl-async under sbcl (tried with 1.2.6),
mostly due wrong types here and there.
The problem with `write-sequence` only manifested itself when using
`(sb-ext:restrict-compiler-policy 'debug 3)`.
Also, using `(handler-case (symbol-function ...) (undefined-function () nil))`
is perhaps not the best solution when there's `fboundp` function (unless
one cares about possible `fmakunbound` race which is rather unlikely thing
to happen in production code).
When tcp-connect failed in the middle of it due to a bug (such as wrong data
value type being passed), some strange errors were happening when
exiting event loop due to presence of objectless handles.