Cache improvements

[jrmaddison]

1. Values, provided by the replace_map when generating cache entries, are now included in the key.
2. Updating the state of a value via var_update_state now clears associated cache entries.
3. Validity of cache entries is checked every time before use with extra var_update_caches calls. This catches all cache invalidation visible to tlm_adjoint -- which with the Firedrake backend includes any updates to dat versions, even those that do not have associated var_update_state calls.

A better alternative to 3 is to make hashing cheap and then simply check the caches every time -- but this needs #183 in order to avoid leaking memory when applying checkpointing.