Remove fil_node_t::sync_event.

I had a discussion with kernel fellows and they said it's safe to call
fsync() simultaneously at least on VFS and ext4. So initially I wanted
to disable check for recent Linux but than I realized code is buggy.

Consider a case when one thread is inside fsync() and two others are
waiting inside os_event. First thread after fsync() calls os_event_set()
which is a broadcast! So two waiting threads will awake and may call
fsync() at the same time.

One fix is to add a notify_one() functionality to os_event but I decided
to remove incorrect check completely. Note, it works for one waiting
thread but not for more than one.

IMO it's ok to avoid existing bugs but there is not too much sense in
avoiding possible(!) bugs as this code does.

fil_space_t::is_in_rotation_list(), fil_space_t::is_in_unflushed_spaces():
Replace redundant bool fields with member functions.

fil_node_t::needs_flush: Replaces fil_node_t::modification_counter and
fil_node_t::flush_counter. We need to know whether there _are_ some
unflushed writes and we do not need to know _how many_ writes.

fil_system_t::modification_counter: Remove as not needed.
Even if we needed fil_node_t::modification_counter, every file
could have its own counter that would be incremented on each write.

fil_system_t::modification_counter is a global modification counter
for all files. It was incremented on every write. But whether some
file was flushed or not is an internal fil_node_t deal/state and
this makes fil_system_t::modification_counter useless.

Closes #1061