Avoid a copy when inserting a mempool entry

Summary:
We already store a unique_ptr, so we can leverage this to avoid the copy.
To avoid creating new copies the copy constructor operator is explicitely deleted.
A move constructor also needs to be created as this is used in the tests.

Note that we don't need assignment operators because the class has const
members.

This will make it easier to reason about when we later update to a RCUPtr, because it uses
an atomic and cannot be copied.

Depends on D13443.

Test Plan:

ninja all check-all

Run the asan build;

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D15147