Do not cache too big byte buffers and too big writeRequestCtx objects,
since it is cheaper to re-create them instead of wasting RAM for their caching.
This reverts 7f6f350ee1
Previously targets with big number of metrics and/or labels could generated too big buffers,
which then could be re-used when scraping targets with small number of metrics.
This resulted in memory waste.
Now big buffers are used only for targets with big number of metrics / labels,
while small buffers are used for targets with small number of metrics / labels.
