node: Add reference to mempool in NodeContext

Summary:
Currently it is an alias to the global ::mempool and should be used as
follows.

• Node code (validation and transaction relay) can use either ::mempool or node.mempool, whichever seems a better fit.
• RPC code should use the added convenience getter EnsureMempool, which makes sure the mempool exists before use. This prepares the RPC code to a future where the mempool might be disabled at runtime or compile time.
• Test code should use m_node.mempool directly, as the mempool is always initialized for tests.

This is a partial backport of Core PR17407 : https://github.com/bitcoin/bitcoin/pull/17407/commits/fac07f2038a3ccd5edadc6e6122c02fa30e697bd

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D6437