Zerofill m_fill for safe fiber recycling, fixes #753

Reset initialization vector also

Added destructor support

Changed destructor to function and pointer

do not expose TaskDestructor

Using static struct for destructors

Checking for empty FLSInit
Additional precautions for possible access violations

Remove unsafe TaskLocal destructor

Remove bugs
Merge pull request #754 from etcimon/tls-safety

Implement safe fiber recycling, fixes #753