Adds support for the Transparent Application Failover Callback.
The php_oci_connection struct got a char* added which will contain the
callback function, it should be set to PHP_OCI_TAF_DISABLE_CALLBACK at
the end of a php request for permanent connections so that, if a TAF
callback occurs, no userspace function will be called.
Maybe add support for registering object functions (via array),
currently the register function only accepts a string. I didn't know how
to implement it correctly. As a failover occurs very rarely it might be
better to not keep the cache when saving the zend_fcall_info.
Things to do
[ ] config.m4 needs to compile oci8_failover.c
[ ] Check if correctly implemented (especially for multithreading)
[ ] Add support for registering callback function via array
