Bug 943744 (part 1) - Remove StringInfo::length.  r=till.

--HG--
extra : rebase_source : 4ac0be283e35bd878ad4410b805548255fcd8cee
Bug 943744 (part 2) - Represent the sizes in StringInfo more compactly.  r=till.

--HG--
extra : rebase_source : 799693bac2beb4609daf55fc5e8cbe19adff3ed8
Bug 943744 (part 3) - Re-use the |strings| table from the zone with the most strings when computing totals.  r=till.

--HG--
extra : rebase_source : 3fa7b4249526ffcfbadd108aa55e7e9e5f0c8ebf