Use &self instead of mut self when provider sends txs.

[adlrocha]

Related to #400 and tracked here.

Making this mut will lead to methods that require sending txn to use mut self when using the provider, which might force owner of this struct to use mut self, i.e. fendermint top down provider might encounter this issue. If mut is not necessary, we can just stick with self, else callers might have to use RWLock for this.