MINOR: shctx: allow consumers to customize eviction strategy

MINOR: shctx: allow consumers to customize eviction strategy

This introduces a make_room() callback that consumers can provide in
order to implement more specific eviction strategies. To be used in the
HTTP cache code. This goes in hand with a new shctx_row_truncate()
function that allows consumers to truncate a row, which can be used in
the make_room() callback.