Fork-safe read access

[arsenius7]

It's unsafe to open same H5 file with libsonata after forking.
In the best case, it can end up with errors like that one:

#005: H5C.c line 3574 in H5C_protect(): can't load entry
    major: Symbol table
    major: Object cache
    minor: Protected metadata error
    minor: Unable to load metadata into cache

In the worst case, it can lead to undefined behavior.

This nice trait is inherited from libhdf5; if possible we need to find a way to protect library users from this, or at least make the error more self-explanatory.

[mgeplf]

Sadly we don't have a lot of control on how calling process' handle themselves.

However, it would be neat to be able to at least issue a warning if a fork() happens without an exec.
The only (gross) way I can think of this is to save the PID when libsonata is initialized, and periodically check that it hasn't changed; an exec() would clear this, iirc (to be checked).

This doesn't seem like an elegant solution, though.

[mgeplf]

@GianlucaFicarelli or @matz-e; do you guys have any thoughts on this? Anyone you recommend I talk to?