Turn the global mempool into a shared pointer

[jnewbery]

Global mempool is defined here: https://github.com/bitcoin/bitcoin/blob/8237889e8d0fb7542669a9098516c96da91913f0/src/validation.cpp#L130

It should be changed to a shared pointer (and renamed g_mempool) so that it isn't initialized until runtime. That would allow us to remove the code added in bitcoin#6889.

[maflcko]

Have you seen bitcoin#17407 ?

Also, it should probably be a unique ptr to keep everyone sane

[jnewbery]

D'oh. Yes I even commented on it. Big concept ACK!