Deprecate the zcashd -optimize-getheaders configuration option

[LarryRuane]

PR #4624 improved initial block download (IBD) performance for release 4.3.0, but there was a small concern that the change could have bad side-effects (for example, cause IBD to stall). Purely as a defensive measure, in case a user ran into a problem with the change, that PR also added an option to disable the new behavior. The default was -optimize-getheaders (or -optimize-getheaders=1), that is, enable the improvement, but it could be disabled with -nooptimize-getheaders(or-optimize-getheaders=0`).

After another release or two, we'll be confident in the fix, and this option can be removed (just for clean-up). This ticket is to remind us to do that. It should probably be removed in 4.4.0 or 4.5.0.

[daira]

We should probably include this in 4.7.0.