refactor: Move CTxMemPoolEntry class to its own module

This change nukes the policy/fees->mempool circular dependency.

Easy to review using `diff --color-moved=dimmed-zebra`.

test/sanitizer_suppressions/ubsan

@@ -53,6 +53,7 @@ unsigned-integer-overflow:policy/fees.cpp
 unsigned-integer-overflow:prevector.h
 unsigned-integer-overflow:script/interpreter.cpp
 unsigned-integer-overflow:txmempool.cpp
+unsigned-integer-overflow:txmempool_entry.cpp
 implicit-integer-sign-change:compat/stdin.cpp
 implicit-integer-sign-change:compressor.h
 implicit-integer-sign-change:crypto/
@@ -62,6 +63,7 @@ implicit-integer-sign-change:script/bitcoinconsensus.cpp
 implicit-integer-sign-change:script/interpreter.cpp
 implicit-integer-sign-change:serialize.h
 implicit-integer-sign-change:txmempool.cpp
+implicit-integer-sign-change:txmempool_entry.cpp
 implicit-signed-integer-truncation:crypto/
 implicit-unsigned-integer-truncation:crypto/
 shift-base:arith_uint256.cpp