If the hash is not `noexcept`, `unorderd_map` has to assume that it can throw an exception. Thus when rehashing care needs to be taken. libstdc++ solves this by simply caching the hash value, which increases memory of each node by 8 bytes. Adding `noexcept` prevents this caching. In my experiments with `-reindex-chainstate -stopatheight=594000`, memory usage (maximum resident set size) has decreased by 9.4% while runtime has increased by 1.6% due to additional hashing. Additionally, memusage::DynamicUsage() is now more accurate and does not underestimate.

|                                       | runtime h:mm:ss | max RSS kbyte |
|---------------------------------------|-----------------|--------------|
| master                                |         4:13:59 |      7696728 |
| 2019-09-SaltedOutpointHasher-noexcept |         4:18:11 |      6971412 |
| change                                |          +1.65% |       -9,42% |

Comparison of progress masters vs. 2019-09-SaltedOutpointHasher-noexcept
![out](https://user-images.githubusercontent.com/14386/65541887-69424e00-df0e-11e9-8644-b3a068ed8c3f.png)