thrift: capture "this" explicitly in lambda
thrift: 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: <e766d6c8992d89f80fb23cc209e2353956e4973b@scylladb.com>

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