SQLCipher integration with API for storing Session Pro revocations and account seed

[Doy-lee]

This introduces SQLCipher, building on from https://github.com/mpretty-cyro/libsession-util/tree/feature/initial-database# The database is exposed under the session::database::Connection interface and we also in addition to that introduce a overarching managing class dubbed session::core::Core that manages requests going into the database, caching queries before insertion and returning results from caches instead of the DB if it hasn't been invalidated yet.

The intended usage pattern of the new code looks something like this:

  #include <sodium/randombytes.h>
  #include <session/core.hpp>

  int main() {
    session::core::Core core = {};

    // Optionally create/open the DB to persist state to. If this step is skipped the core will only
    // maintain libsession state (like the user's long term seed or the pro revocation list) in
    // runtime memory and will be lost on shutdown. Persisting user state is then left to the
    // integrating application's discretion.
    try {
      // Generate the encryption key for the DB (if you had a pre-existing DB this is where you
      // would load the key to pass in).
      session::cleared_array<48> db_enc_key = {};
      randombytes_buf(db_enc_key.data(), db_enc_key.size());

      core.open_db(":memory:", db_enc_key);
    } catch (const std::exception& e) {
      // ... error handling
    }

    // Update the revocation list stored in Core (if the DB was opened successfully, this will also
    // persist the revocation list to the DB for example).
    //
    // In a production application you would sleep on an event loop responsible for dispatching and
    // receiving the revocation list queries and call this function to update the revocation list
    // that is cached and the DB
    if (core.pro_update_revocations(...)) { ... }

    // Interfacing code calls this API to check if the specific proof in question is revoked or not
    if (core.pro_proof_is_revoked(...)) { ... }
  }