journal: coalesce ftruncate()s in 250ms windows

journal: coalesce ftruncate()s in 250ms windows

Prior to this change every journal append causes an ftruncate() for the
sake of inotify propagation of the mmap-based writes.

With this change the notification is deferred up to ~250ms, coalescing
any repeated journal writes during the deferred period into a single
ftruncate().  The ftruncate() call isn't free and doing it on every
append adds unnecessary overhead and latency in the journald event loop.

Introduces journal_file_enable_post_change_timer() which manages a
timer on the provided sd-event instance for scheduling coalesced
ftruncates.  The ftruncate() behavior is unchanged unless
journal_file_enable_post_change_timer() is called on the JournalFile.

While not a tremendous improvement, profiling systemd-journald event loop
latencies using instrumentation as introduced by 34b8751 it was observed that
coalescing the ftruncates was low-hanging fruit worth pursuing.

Note orders 12 and 13 shifting left into order 11 and order 6 dipping into
order 5:

Unmodified:
     log2(us)   1 2 3  4 5  6   7   8  9   10 11   12   13 14 15 16 17 18 19
                -----------------------------------------------------------
[10685.414572]  0 0 0  0 38 602 61  2  290 60 1643 2554 13 1  4  1  0  0  1
[10690.415114]  0 0 0  0 0  646 54  7  309 44 2073 2148 17 1  3  0  0  0  1
[10695.415509]  0 0 0  0 1  650 73  3  324 37 2071 2270 9  0  0  1  0  1  0
[10700.416297]  0 0 0  0 0  659 50  4  318 38 2111 2152 6  0  1  0  0  1  1
[10705.417136]  0 0 0  0 2  660 48  4  320 38 2129 2146 12 1  1  0  0  1  1
[10710.489114]  0 0 0  0 0  673 38  3  321 37 1925 2339 7  0  0  0  0  1  1
[10715.489613]  0 0 0  0 3  656 64  8  317 48 2365 2007 7  0  0  0  0  0  1

Coalesced:
     log2(us)   1 2 3  4 5  6   7   8  9   10 11   12   13 14 15 16 17 18 19
                -----------------------------------------------------------
[ 6169.161360]  0 0 0  1 24 786 54  11 389 24 4192 771  6  4  0  0  1  0  1
[ 6174.161705]  0 0 0  1 18 800 35  6  380 27 3977 893  3  1  0  0  1  0  1
[ 6179.162741]  0 0 0  1 28 768 51  4  391 16 3998 831  5  3  0  0  0  0  2
[ 6184.162856]  0 0 0  0 19 770 60  2  376 26 3795 1004 9  5  1  0  1  0  1
[ 6189.163279]  0 0 0  0 28 761 49  7  372 27 3729 1056 3  2  0  0  1  0  1
[ 6194.164255]  0 0 0  0 25 785 49  7  394 19 3996 908  6  3  2  0  0  0  1
[ 6199.164658]  0 0 0  0 29 797 35  5  389 18 3995 898  3  4  1  1  1  0  1

The remaining high-order delays are a result of the synchronous fsyncs in
systemd-journald, beyond the scope of this commit.