alloc: properly allocate memory handler per thread

[bouk]

Right now the memory handler is allocated once, for the first thread
that happens to call it. If pthread_getspecific is then called from
another thread, the memory handler is not allocated (because
pthread_once is only called for all threads, not per thread) and jq
aborts because it thinks memory allocation failed.

What we need to do instead is call pthread_key_create from the
initializer, and then allocate a memory handler for every thread when it
sees there's none yet.

I also removed the atexit stuff, and instead use the destructor
function you can pass into pthread_key_create.

I figured this out by looking at this example: https://www.ibm.com/support/knowledgecenter/ssw_ibm_i_74/rzahw/rzahwe21rx.htm and the pthread docs

[coveralls]

Coverage increased (+0.4%) to 84.517% when pulling 0be689b on bouk:fix-threads into ccc79e5 on stedolan:master.