osd: EC Optimizations proc_master_log boundary case bug fixes

osd: EC Optimizations proc_master_log boundary case bug fixes

Fix a couple of bugs in proc_master_log for optimized EC
pools dealing with boundary conditions such as an empty
log and merging two logs that diverge from the very first
entry.

Refactor the code to handle the boundary conditions and
neaten up the code.

Predicate the code block with if (pool.info.allows_ecoptimizations())
to make it clear this code path is only for optimized EC pools.

Signed-off-by: Bill Scales <bill_scales@uk.ibm.com>
(cherry picked from commit 1b44fd9991f5f46b969911440363563ddfad94ad)