Intro* INSERT statements could face a non-existant block when MCOL-498 feature
is enabled. writeRow() guard blocks was supposed to proactively create empty
blocks. The pre-patch logic failed when first value in the block has been
removed by DELETE and this overwrites the whole valid block with empty magics.
This patch moves proactive creation logic into allocRowId().
* DML writes for multi-block dictionary (blob) now works
* PrimProc fixed so that the first block in multi-block is read
correctly
* Performance optimisation (removed string copy into stack) for new
dictionary entries
