Remove RPC FilterManager mutability

[knkski]

Because the FilterManager manages itself with interior mutability, references to FilterManager don't need to be mut. This PR removes the unnecessary mutability.

[dplumb94]

Got an error on build:

error: unknown start of token: `
   --> src/client.rs:225:95
    |
225 |     pub fn send_request<T, U>(&mut self, msg_type: Message_MessageType, msg: &T) -> Result<U, `>
    |                                                                                               ^
help: Unicode character '`' (Grave Accent) looks like ''' (Single Quote), but it is not
    |
225 |     pub fn send_request<T, U>(&mut self, msg_type: Message_MessageType, msg: &T) -> Result<U, '>
    |                                                                                               ^

error: aborting due to previous error

error: Could not compile `rpc`.

To learn more, run the command again with --verbose.

Not sure if that is part of your PR or not

[knkski]

@dplumb94: I don't think that's due to this PR, since that's coming from a file that's unaffected by this PR. Can you try doing git reset --hard HEAD on this branch and seeing if that fixes it?