osd: tolerate misordered TMAP updates

osd: tolerate misordered TMAP updates

The previous tmap implementation requires that the update stream be
sorted or else it will behave erratically (by placing new keys in the
map out of order).  This can cause very strange failures: reads may
appear to return the correct result initially, but once intervening
keys are remove they will not... depending on how read is implemented
on the client side.

Fix this by doing the optimized updates initially, but falling back to
a slow implementation if an unsorted update is detected.  It is slow,
but such updates are rare.

Signed-off-by: Sage Weil <sage@inktank.com>