Take apart the SENDME cell specific code and put it in sendme.{c|h}. This is
part of prop289 that implements authenticated SENDMEs.

Creating those new files allow for the already huge relay.c to not grow in LOC
and makes it easier to handle and test the SENDME cells in an isolated way.

This commit only moves code. No behavior change.

Signed-off-by: David Goulet <dgoulet@torproject.org>

Fixes bug 30263; bugfix on 0.4.0.1-alpha.

Closes ticket 30234.

Only set rate_limited in hs_pick_hsdir() if rate_limited_count or responsible_dirs_count is greater than 0

"ours" merge, to avoid taking any changes from PR 792 in 0.3.4.
(We already merged PR 791 for 29665 into 0.3.4.)