Suppress conversion warnings reported by MSVC.

[SVN r86462]
Support older versions of OpenSSL that don't supply SSL_CTX_clear_options.

[SVN r86463]
Add a note to clarify that concurrent invocation is not guaranteed for handlers in different strands.

[SVN r86464]
Fix typos in spawn() documentation.

[SVN r86465]
Updated revision history.

[SVN r86468]