PERFORMANCE: avoid repeated serialization of terms

[jacoscaz]

We can improve write performance levels by avoiding duplicate and unnecessary term serializations. Terms can simply be serialized once and their serialized versions can then be concatenated around based on each index's permutation of terms. Whether the ROI of doing this is high enough to justify the effort is TBD.

[jacoscaz]

Closing this as ideas for performance improvements are now tracked as cards in https://github.com/beautifulinteractions/node-quadstore/projects/9 .