include/denc: use pair<const K,V> in range-based for loop

include/denc: use pair<const K,V> in range-based for loop

map<K,V>::value_type is pair<const K, V>, so if we use range-based for
loop when iterating through a map, we should use pair<const K,V> instead
of pair<K,V>, the latter also compiles, but it might create a temporary
object of pair<K,V> from pair<const K,V>. GCC-11 complains at seeing
this:

../src/include/denc.h:1002:21: warning: loop variable ‘e’ of type ‘const T&’ {aka ‘const std::pair<OSDPerfMetricQuery, OSDPerfMetricReport>&’} binds to a tem\
porary constructed from type ‘const std::pair<const OSDPerfMetricQuery, OSDPerfMetricReport>’ [-Wrange-loop-constru
ct]
 1002 |       for (const T& e : s) {
      |                     ^

this change

* use the value_type of container in `maplike_details<Container>`,
  so we can avoid the overhead of creating temporay objects when
  encoding a map
* define denc_traits for std::pair<const A, B> as well, so the elements
  of a map can be encoded using denc facility

Signed-off-by: Kefu Chai <kchai@redhat.com>
(cherry picked from commit c828ce29400a1eea4b223229b36b3a092eda6139)