loading_cache: prepend this-> to method calls on captured this
loading_cache: prepend this-> to method calls on captured this

To make gcc 6.3 happy.

Signed-off-by: Botond Dénes <a584fa4d75a0a85d721f44fb6e06315a2853b0b6@scylladb.com>
Message-Id: <849402e20a1ffa6f603eff4fe295981a94b9ca79.1507282527.git.a584fa4d75a0a85d721f44fb6e06315a2853b0b6@scylladb.com>

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