>>> backports bitcoin/bitcoin@4536148

Add a pool for locked memory chunks, replacing LockedPageManager.

This is something I've been wanting to do for a long time. The current
approach of locking objects where they happen to be on the stack or heap
in-place causes a lot of mlock/munlock system call overhead, slowing
down any handling of keys.

Also locked memory is a limited resource on many operating systems (and
using a lot of it bogs down the system), so the previous approach of
locking every page that may contain any key information (but also other
information) is wasteful.

>>> backports bitcoin/bitcoin@6567999

```
getmemoryinfo
Returns an object containing information about memory usage.

Result:
{
  "locked": {               (json object) Information about locked
memory manager
    "used": xxxxx,          (numeric) Number of bytes used
    "free": xxxxx,          (numeric) Number of bytes available in
current arenas
    "total": xxxxxxx,       (numeric) Total number of bytes managed
    "locked": xxxxxx,       (numeric) Amount of bytes that succeeded
locking. If this number is smaller than total, locking pages failed at
some point and key data could be swapped to disk.
  }
}

Examples:
> bitcoin-cli getmemoryinfo
> curl --user myusername --data-binary '{"jsonrpc": "1.0",
"id":"curltest", "method": "getmemoryinfo", "params": [] }' -H
'content-type: text/plain;' http://127.0.0.1:8332/
```