Reduce number of String allocation in Stratum worker
Reduce number of String allocation in Stratum worker

Currently we allocate a string in the heap per each worker check (not even
read) and clone message in write. This pr introduces a buffer which is
used for a round of workers check and remove clone in write_message.
Read optimization reduces number of allocations by factor of N (where N
is a number of workers), if N = 1 number of allocations doesn't change.

yeastplume/grin,yeastplume/grin