Make set_ps_display faster and easier to use

While doing some benchmarking of some fast-to-execute queries, I see
that set_ps_display() popping up on the profiles.  Looking a little
deeper, there are some inefficiencies in there that we could fix.

For example, the following is pretty poor:

strlcpy(ps_buffer + ps_buffer_fixed_size, activity,
        ps_buffer_size - ps_buffer_fixed_size);
ps_buffer_cur_len = strlen(ps_buffer);

We already know the strlen of the fixed-sized part, so why bother
doing strlen on the entire thing?  Also, if we did just do
strlen(activity), we could just memcpy, which would be much faster
than strlcpy's byte-at-a-time method of copying.

Adjusting that lead me to notice that we often just pass string
constants to set_ps_display(), so we already know the strlen for this
at compile time. So maybe we can just have set_ps_display_with_len()
and then make a static inline wrapper that does strlen() so that when
the compiler can figure out the length, it just hard codes it.

After doing that, I went over all usages of set_ps_display() to see if
any of those call sites knew the length already in a way that the
compiler wouldn't be able to deduce. There were a few cases to adjust
when setting the process title to contain the command tag.

After fixing up the set_ps_display()s to use set_ps_display_with_len()
where possible, I discovered some not so nice code which appends "
waiting" onto the process title. Basically, there's a bunch of code
that looks like this:

const char *old_status;
int len;

old_status = get_ps_display(&len);
new_status = (char *) palloc(len + 8 + 1);
memcpy(new_status, old_status, len);
strcpy(new_status + len, " waiting");
set_ps_display(new_status);
new_status[len] = '\0'; /* truncate off " waiting" */

Seeing that made me wonder if we shouldn't just have something more
generic for setting a suffix on the process title.  I came up with
set_ps_display_suffix() and set_ps_display_remove_suffix().  The above
code can just become:

set_ps_display_suffix("waiting");

then to remove the "waiting" suffix, just:

set_ps_display_remove_suffix();

I considered adding a format version to append the suffix as there's
one case that could make use of it, but in the end, decided it might
be overkill, so I left that code like:

char buffer[32];

sprintf(buffer, "waiting for %X/%X", LSN_FORMAT_ARGS(lsn));
set_ps_display_suffix(buffer);

I don't think that's terrible enough to warrant making a va_args
version of set_ps_display_suffix(), especially for just 1 instance of
it.

I also resisted making set_ps_display_suffix_with_len(). The new code
should be quite a bit
faster already without troubling over that additional function.

I've attached the patch.

David