Use waste metric for deciding which selection to use

Instead of always choosing BnB if it finds a solution, always do both
BnB and KnapsackSolver and choose the one which has the least waste.

[doc] improve RBF documentation + style cleanups after refactors

Document a few non-obvious things and delete no-longer-relevant comments
(e.g. about taking a lock that we're already holding).
No change in behavior.

Bitcoin Core 0.19.1 release candidate 2

Tree-SHA512: 7ba1efdc24835405562b5ff710ed4223d7e4463dffa58721cc9e0f0f0ba90925e59719cb68b31f95cfc632a73ca9a5573e762167eed264c66323828a397b9127

Bitcoin Core 0.19.1 release candidate 1

Tree-SHA512: 8438f992d0c39315a4da4c3f8ab5c92acefada4b0ba5a5cec3775dea2541492d386bf4e7c9e76e1494a6d4cf16a9205287d27ffd23d9c3056f213d733605eeee

Bitcoin Core 0.19.0.1 final

Tree-SHA512: d53a2bfa5bc589e7130cf8cd1627083ad7c72a197cbd80754c9dcf78e3775021fbd6426c5762f5f3e52e4b23ed06387c0be224914e0f98d14f3f7e51ee66c809

Bitcoin Core 0.19.0 final

Tree-SHA512: 3d071f93f6fc5b30ccfb3ca34d7e050370d2d6201aca892503a4300a761116d69ceed7c770283581fcce0ce78ff7c0a6820db8600877d09fba9ed0cd488f4bf7

Bitcoinc Core 0.19.0 release candidate 3

Tree-SHA512: 84fae5ac33200ba995384f87d43c3d0b138043e47094dd62731e1558fdc74a8df283046437f1d0c228cdee519bbf9c167a1696a7a4d915f48da97c9524f320c2

Bitcoin Core 0.19.0 release candidate 2

Tree-SHA512: 6e36d626215d5e66796bd3935037eef89ae810e4855e321f9f37b90c3be0b61de21cd655f23bc6a29f41038b5179d92578ab19ab89ccd525d9389ab868456827

Bitcoin Core 0.19.0 release candidate 1

Tree-SHA512: c8fa2ce869402f5c8e5e8fd8c835cce5d1ba482cb784613ea074a98e7a1035ab64f832f4232d745fc7c79966e5201be4c843af45f36284653e793965bb415e0e

Bitcoin Core 0.17.2 final release

Tree-SHA512: 2c7a2189888eb27563ada133fb3decfa67804cbabd729f0839e40185d9e40df82d5450f7fdc955fe87b0d94c5f211b52c02695e7b008989cf2487bdb66b765f5