pool: Add and use MemPoolOptions, ApplyArgsManOptions

Reviewers: Note that CTxMemPool now requires a non-defaulted
CTxMemPool::Options for its constructor. Meaning that there's no need to
worry about a stray CTxMemPool constructor somewhere defaulting to
something incorrect. All instances of CTxMemPool construction are
addressed here in this commit.

We set options for CTxMemPool and construct it in many different ways. A
good example can be seen in how we determine CTxMemPool's check_ratio in
AppInitMain(...).

1. We first set the default based on chainparams's
   DefaultConsistencyChecks()
2. Then, we apply the ArgsManager option on top of that default
3. Finally, we clamp the result of that between 0 and 1 Million

With this patch, most CTxMemPool construction are along the lines of:

    MemPoolOptions mempool_opts{...default overrides...};
    ApplyArgsManOptions(argsman, mempool_opts);
    ...hard overrides...
    CTxMemPool pool{mempool_opts};

This "compositional" style of building options means that we can omit
unnecessary/irrelevant steps wherever we want but also maintain full
customizability.

For example:

- For users of libbitcoinkernel, where we eventually want to remove
  ArgsManager, they simply won't call (or even know about)
  ApplyArgsManOptions.

- See src/init.cpp to see how the check_ratio CTxMemPool option works
  after this change.

A MemPoolOptionsForTest helper was also added and used by tests/fuzz
tests where a local CTxMemPool needed to be created.

The change in src/test/fuzz/tx_pool.cpp seemingly changes behaviour by
applying ArgsManager options on top of the CTxMemPool::Options defaults.
However, in future commits where we introduce flags like -maxmempool,
the call to ApplyArgsManOptions is actually what preserves the existing
behaviour. Previously, although it wasn't obvious, our CTxMemPool would
consult gArgs for flags like -maxmempool when it needed it, so it
already relied on ArgsManager information. This patchset just laid bare
the obfuscatory perils of globals.

[META] As this patchset progresses, we will move more and more
       CTxMemPool-relevant options into MemPoolOptions and add their
       ArgsMan-related logic to ApplyArgsManOptions.

1 files changed, 3 insertions, 2 deletions

diff --git a/src/txmempool.cpp b/src/txmempool.cpp
index 65c8b4ea60..84088aead6 100644
--- a/src/txmempool.cpp
+++ b/src/txmempool.cpp
@@ -451,8 +451,9 @@ void CTxMemPoolEntry::UpdateAncestorState(int64_t modifySize, CAmount modifyFee,
     assert(int(nSigOpCostWithAncestors) >= 0);
 }
 
-CTxMemPool::CTxMemPool(CBlockPolicyEstimator* estimator, int check_ratio)
-    : m_check_ratio(check_ratio), minerPolicyEstimator(estimator)
+CTxMemPool::CTxMemPool(const Options& opts)
+    : m_check_ratio{opts.check_ratio},
+      minerPolicyEstimator{opts.estimator}
 {
     _clear(); //lock free clear
 }