Refactor db interface

[jonasseglare]

This PR mainly refactors the code in src/datahike/db.cljc and src/datahike/db/interface.cljc so that datom filtering and evaluation of current datoms from historical ones is moved into the single function datahike.db/post-process-datoms. This makes the code easier to reason about and deduplicates some calls. These changes will also make it easier to move forward with the code in the PR #674 .

I had to introduce a record SearchContext to make it fast enough and resort to Java interop syntax to access fields. Otherwise, it is too slow.

I measured the performance change and this code takes rougly 2% less time in my benchmark which is a small improvement:

TARGET                          ABS TIME (s)   REL TIME
Official Datahike                     49.765       100%
Refactor db interface                 48.666        98%

[whilo]

Looks good. Thanks!