storage_proxy: capture "this" explicitly in lambda
storage_proxy: capture "this" explicitly in lambda

C++20 deprecates capturing this in default-copy lambdas ([=]), with
good reason. Move to explicit captures to avoid any ambiguity and
reduce warning spew.
Message-Id: <950cb746e8a5b37da9110cbf40fb0e242304925f@scylladb.com>

avikivity/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla,avikivity/scylla,scylladb/scylla,avikivity/scylla