Support asynchronous external indexing

[mattgodbolt]

At the moment line buffering is required of the --pipe command, as the whole thing is synchronous. Ideally the external indexer could run in parallel with zindex itself, which would make it impossible to deadlock, and can probably speed up the operation too.

[mattgodbolt]

Will probably have to implement this as some large jq queries are sometimes hitting some buffering limit (e.g. a query I have which has 567481 byte lines, and 6000 byte outputs)

[mattgodbolt]

That turned out to be a bug, so this is just a "would be nice"

[mattgodbolt]

This still seems to happen when (for example) tests run under an IDE (eg CLion)

[mattgodbolt]

(/home/mgodbolt/dev/zindex/tests/ExternalIndexerTest.cpp:21)