New DTrace probes proposal

(Resending since it didn't work the first time. Not sure if attaching a
tar file was the culprit.)

I'd like to propose adding the following probes (some of which came from
Simon) to 8.4.

I think these probe provide very useful data. Although some of the data
can be collected now, the main advantages with probes, among others, are
(1) they are always available and can be enabled only when needed
especially in production (2) different combinations of probes can be
used together to collect interesting data.

They work on OS X Leopard & Solaris now, and hopefully on FreeBSD soon.

Preliminary patch attached along with sample DTrace scripts.

-Robert

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* Probes to measure query time
query-parse-start (int, char *)
query-parse-done (int, char *)
query-plan-start ()
query-plan-done ()
query-execute-start ()
query-execute-done ()
query-statement-start (int, char *)
query-statement-done (int, char *)

* Probes to measure dirty buffer writes by the backend because bgwriter
is not effective
dirty-buffer-write-start (int, int, int, int)
dirty-buffer-write-done (int, int, int, int)

* Probes to measure physical writes from the shared buffer
buffer-write-start (int, int, int, int)
buffer-write-done (int, int, int, int, int)

* Probes to measure reads of a relation from a particular buffer block
buffer-read-start (int, int, int, int, int)
buffer-read-done (int, int, int, int, int, int)

* Probes to measure the effectiveness of buffer caching
buffer-hit ()
buffer-miss ()

* Probes to measure I/O time because wal_buffers is too small
wal-buffer-write-start ()
wal-buffer-write-done ()

* Probes to measure checkpoint stats such as running time, buffers
written, xlog files added, removed, recycled, etc
checkpoint-start (int)
checkpoint-done (int, int, int, int, int)

* Probes to measure Idle in Transaction and client/network time
idle-transaction-start (int, int)
idle-transaction-done ()

* Probes to measure sort time
sort-start (int, int, int, int, int)
sort-done (int, long)

* Probes to determine whether or not the deadlock detector has found a
deadlock
deadlock-found ()
deadlock-notfound (int)

* Probes to measure reads/writes by block numbers and relations
smgr-read-start (int, int, int, int)
smgr-read-end (int, int, int, int, int, int)
smgr-write-start (int, int, int, int)
smgr-write-end (int, int, int, int, int, int)