Merge #11585: addrman: Add missing lock in Clear() (CAddrMan)

Summary:
3ab545d7f addrman: Add missing lock in Clear() (CAddrMan) (practicalswift)

Pull request description:

Add missing lock in `Clear()` (`CAddrMan`).

The variable `vRandom` is guarded by the mutex `cs`.

**Note to reviewers:** Does this look correct? Should the lock cover the entire scope of the method, or should it be limited to cover only `std::vector<int>().swap(vRandom);`?

Tree-SHA512: 8833f31beaed1728fa55b13ddf9e0b8e24e395931497329be2440ce1c5113ff02871707d40830260adabd30c4ea86088f5da5cf8a821150c0d820f50a2ce386a

Backport of Core PR 11585
https://github.com/bitcoin/bitcoin/pull/11585/files

Test Plan: ninja check

Reviewers: deadalnix, Fabien, #bitcoin_abc

Reviewed By: Fabien, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D2945