osdc/ObjectCacher: match reads with their original rx buffers

Consider a sequence like:

 1- start read on 100~200
       100~200 state rx
 2- truncate to 200
       100~100 state rx
 3- start read on 200~200
       100~100 state rx
       200~200 state rx
 4- get 100~200 read result

Currently this makes us crash on

osdc/ObjectCacher.cc: 738: FAILED assert(bh->length() <= start+(loff_t)length-opos)

when processing the second 200~200 bufferhead (it is too big).  The
larger issue, though, is that we should not be looking at this data at
all; it has been truncated away.

Fix this by marking each rx buffer with the read request that is sent to
fill it, and only fill it from that read request.  Then the first reply
will fill the first 100~100 extend but not touch the other extent; the
second read will do that.

Signed-off-by: Sage Weil <sage@inktank.com>
(cherry picked from commit b59f930ae147767eb4c9ff18c3821f6936a83227)
osdc/ObjectCacher: do not merge rx buffers

We do not try to merge rx buffers currently.  Make that explicit and
documented in the code that it is not supported.  (Otherwise the
last_read_tid values will get lost and read results won't get applied
to the cache properly.)

Signed-off-by: Sage Weil <sage@inktank.com>
(cherry picked from commit 1c50c446152ab0e571ae5508edb4ad7c7614c310)