Re-add raw methods, accept leveldb in constructor, minor fixes

[s1na]

tl;dr Changes since the v3.0.0 release had introduced breaking changes (and bugs). This PR makes those changes backwards compatible, and fixes the bugs. ethereumjs-vm passes all tests without any modifications after integrating this PR.

The changes are:

• Re-introduce getRaw, putRaw, delRaw methods (but mark them as deprecated)
• Instead of a DB object, instantiate the trie via a leveldb object (like before). This necessitated some other changes in checkpointTrie and secureTrie.
• Fixed a bug in CheckpointTrie._exitCpMode which passed the wrong object to WriteStream
• The first two changes created a (difficult to track) bug: SecureTrie.copy would copy CheckpointTrie's scratch db and use that as the underlying database, which meant the actual state data wasn't available and this caused quite a few failures.

[coveralls]

Coverage decreased (-0.6%) to 93.575% when pulling d2199a4 on db-methods into 4d918be on master.

[holgerd77]

One comment to update, rest looks good.

[holgerd77]

Looks good now, thanks for the update.

[holgerd77]

@s1na Phew, I am just realizing while scimming through the merged PR history that doing releases on this library got completely lost. 😕 Last release (v3.0.0) was from January 2019.

Do you think it is still worth to do a pre-TypeScript v3.1.0 release or should we rather get the refactoring changes @ryanio is doing right now together and do a bigger v4.0.0 release? Any opinion or preference? Could we in that case branch off for such a release from this PR?

[s1na]

@holgerd77 that's not a bad idea actually. It seems this PR is backwards-compatible (this should be tested again though).

[ryanio]

I realized I removed the @deprecated getRaw/putRaw/delRaw methods during the refactor in #107, so just a note that they won't be included in v4.0.0. If there is a good reason for them to be included please let me know :)