LUDOC-304 tuning: support multiple modify RPCs in parallel

LUDOC-304 tuning: support multiple modify RPCs in parallel

Update the Lustre manual to reflect the support of multiple modify
metadata RPCs in parallel, LU-5319:
https://jira.hpdd.intel.com/browse/LU-5319.

The following sections are added or modified:
- Tuning the Client Metadata RPC Stream
- Reply Reconstruction
- lr_reader debugging utility

Signed-off-by: Gregoire Pichon <gregoire.pichon@bull.net>
Change-Id: I3b4199424b2ca43b94e26ee715ae7cc9d5770080
Reviewed-on: http://review.whamcloud.com/16429
Reviewed-by: Richard Henwood <richard.henwood@intel.com>
Tested-by: Jenkins