Update changelog and upgrade guide

[vweevers]

@ralphtheninja should we:

1. Only link to the abstract-leveldown upgrade guide
2. Copy relevant parts (seeking, rejecting nullish values, serialization of range options, ..)

I'm leaning towards 2, because a large chunk of the abstract-leveldown upgrade guide is about tests.

Also, is there anything else you want to include in the new leveldown major?

[ralphtheninja]

I'm leaning towards 2, because a large chunk of the abstract-leveldown upgrade guide is about tests.

Sounds good to me.

Also, is there anything else you want to include in the new leveldown major?

Not that I can think of. Good work!

[vweevers]

Picking some low hanging fruit:

• Serialize range options of compactRange() #499 (to complete the serialization story)
• Remove sync test abstract-leveldown#300 (comment) (to restore a segfault test removed in abstract-leveldown@6)
• Segfault when calling iterator without open #134 (easy to fix and also about segfaults)
• makeTest utility does not handle done() errors #507 (to make sure we didn't introduce bugs)
• Remove undocumented key argument in ChainedBatch#clear(key)? #496 (to complete the refactored chained batch story)

[vweevers]

For reference, I'm excluding the following items from the changelog:

• Upgrade nan from TODO to 2.11.0 (Update nan to the latest version 🚀 #501)
• Upgrade prebuild-install from TODO to 5.0.0 (Update prebuild-install to the latest version 🚀 #490)
• Upgrade prebuild devDependency from TODO to 8.0.0 (Update prebuild to the latest version 🚀 #502)

As nan is gone and the prebuild things will be replaced by prebuildify.