Compile BDB utility programs as part of compiling zcashd

[daira]

Is your feature request related to a problem? Please describe.

Wallet database corruption happens occasionally. The version of BDB we're using does a bad job of recovering databases automatically when they're opened. (The BDB change logs mention several bugs in db recovery on initialization, fixed in later versions.)

Describe the solution you'd like

When building the bdb dependency, also build the utilities described here. Put them somewhere where users won't trip over them (i.e. not in src/), but that allows us to support a user to attempt to recover their wallet without having to perform an extra step to download the utilities and build them. This should not increase overall build times by much, I think.

Note that the utilities have to be from exactly BDB version 6.2.23, otherwise they're only likely to fail and/or corrupt the wallet.dat further.

Alternatives you've considered

Ideally we'd switch to a more reliable database, but that is a longer-term task. I have seen cases where using the utilities (specifically, using db_recover on a corrupted copy of the wallet db, moving it back to wallet.dat and restarting the node with -reindex) has been successful.