storage_proxy: implement custom speculative retry strategy
storage_proxy: implement custom speculative retry strategy

User may specify time after which speculative retry should happen
instead of relying on cf statics. Use provided value in speculative
executor.

Message-Id: <5e8cfadb9c413c75334aa409089fe387af504a3f@scylladb.com>

scylladb/scylla,raphaelsc/scylla,duarten/scylla,kjniemi/scylla,duarten/scylla,scylladb/scylla,scylladb/scylla,kjniemi/scylla,avikivity/scylla,raphaelsc/scylla,kjniemi/scylla,scylladb/scylla,avikivity/scylla,duarten/scylla,raphaelsc/scylla,avikivity/scylla