Improve resilience against memory attacks

[droidmonkey]

Type of change

• ✅ Refactor (significant modification to existing code)

Description and Context

To reduce residual fragments of secret data in memory after
deallocation, this patch replaces the global delete operator with a
version that zeros out previously allocated memory. It makes use of
the new C++14 sized deallocation, but provides an unsized fallback
with platform-specific size deductions.

This change is only a minor mitigation and cannot protect against
buffer reallocations by the operating system or non-C++ libraries.
Thus, we still cannot guarantee all memory to be wiped after free.

As a further improvement, this patch uses libgcrypt and libsodium
to write long-lived master key component hashes into a secure
memory area and wipe it afterwards.

Testing strategy

Testing as a major part of 2.4.2

Checklist:

• ✅ I have read the CONTRIBUTING document. [REQUIRED]
• ✅ My code follows the code style of this project. [REQUIRED]
• ✅ All new and existing tests passed. [REQUIRED]
• ✅ I have compiled and verified my code with -DWITH_ASAN=ON. [REQUIRED]
• ✅ My change requires a change to the documentation, and I have updated it accordingly.
• ✅ I have added tests to cover my changes.

[phoerious]

Please have another look. It's ready to ship from my side.

[droidmonkey]

We need to include "libsodium" as a required dependency. So need to update INSTALL.MD and the Wiki.

[droidmonkey]

Looks good to me. Do we need to add "Alloc.cpp" to all the intermediate libraries? You added it to Browser but I would think it would come as part of linking against Core.

[phoerious]

It should be enough to build it into the main library as it's applied during link time. I added it to browser, because it didn't link against the core lib, but the core lib is linking against browser, of course. So it's actually redundant here. I did add it to the proxy, though, since that one's a standalone binary that doesn't link against core.