Attempt to make D:OD fork safe

[aklaswad]

This commit add these functions to D:OD, when forked;

• Reset all transaction status in child process
• Destroy and forget all cached DBI handles safely

notes:

• Works only when POSIX::AtFork was found
• Basic logic was taken from Cache::Memcached::Fast::Safe, and some are
  taken from Teng/DBIx::Skinny

Disclaimer:

• Tested only on OSX / Ubuntu with SQLite and MySQL.
• I don't know ( forgot? :) ) how to write tests using real mysql instance for D:OD... test which I used is here https://gist.github.com/aklaswad/edf8659d876b082e8c18ca4619f4e909

[coveralls]

Coverage decreased (-0.3%) to 83.41% when pulling 42e1f86 on Peatix:fork-safe into 42d1546 on sixapart:master.

[aklaswad]

Thanks for better fix!