cross-process locks: make them optional

[bnjbvr]

The crypto-store cross-process lock was optional:

matrix-rust-sdk/crates/matrix-sdk/src/encryption/mod.rs

Line 1743 in 40c6c33

if let Some(lock) = self.client.locks().cross_process_crypto_store_lock.get() {

Newer developments added a cross-process lock to both the media store and the event cache store. They're mandatory, which means they're going to cause lots of writes and additional churn for no good reasons, on some platforms that don't require it (e.g. Android, where it will affect battery consumption as well).

The intent of this issue is to make both these cross-process locks optional, for both stores, in the same way the crypto-store cross-process lock is optional.

cc @Hywan