input: introduce helper for buffered submission of sample data

input: introduce helper for buffered submission of sample data

Input modules often find themselves in the situation where sample data
was received and could be sent to the session bus, but submission should
get deferred to reduce the number of send calls and provide larger data
chunks in these calls. Introduce common support code for buffered sample
data submission (both logic and analog), provide a simple alloc, submit,
flush, and free API.