Subject: initializing subtree with invalid syntax crashes ns-slapd
Description: When an import is executed using a task mechanism,
slapi_task_log_notice is called for logging, where task_log field
points the memory storing the log messages. If multiple log
messages were logged by multiple worker threads simultaneously,
there was a chance that the address of the log message was switched
by realloc while the other threads were accessing the old address.
This patch introduces task_log_lock per task to protect task_log.
Note: slapi_ch_malloc and its friends never return NULL. They rather
exits. Thus, to avoid the confusion which may look leaking the
lock, I eliminated 2 error returns from slapi_task_log_notice.