core: move tx indexer to its own file

[rjl493456442]

This pull request moves all the transaction indexing functions to a separate txindexer.go file and define a txIndexer structure as a refactoring.

For reviewers, the core functions(indexing/unindexing) are not changed, just the refactor.

[holiman]

LGTM

[holiman]

linter complains

core/txindexer.go:220:2: S1023: redundant `return` statement (gosimple)
	return

[holiman]

--- FAIL: TestEthClient (0.14s)
    --- FAIL: TestEthClient/StatusFunctions (0.00s)
        ethclient_test.go:481: unexpected progress: &{0 2 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0}
FAIL

???