Bug 4718: Support filling raw buffer space of shared SBufs (#64)

Bug 4718: Support filling raw buffer space of shared SBufs (#64)

SBuf::forceSize() requires exclusive SBuf ownership but its precursor
SBuf::rawSpace() method does not guarantee exclusivity. The pair of
calls may result in SBuf::forceSize() throwing for no good reason.

New SBuf API provides a new pair of raw buffer appending calls that
reduces the number of false negatives.

This change may alleviate bug 4718 symptoms but does not address its
core problem (which is still unconfirmed).

This is a Measurement Factory project.