Rewrapped a comment..
Some slight non-progress with alt_snapshot_node_t, implementing constructors, using std::map for children_.
Got alt_snapshot_node_t be in an intrusive list alt_cache_t::snapshot_nodes_by_block_id, nothing is actually constructing them or using them yet.
Partly implemented alt_snapshot_node_t logic.  (Parent/child constructor logic, which is a big deal, is missing.)
Updated an RSI comment.
Merge branch 'sam_alt' into sam_alt_snap
Made alt_snapshot_node_t use a current_page_acq_t.
Made alt_snapshot_node_t constructor take current_page_acq argument.
Made use of current_page_acq() helper function.
Made snapshot_subtree not have to wait for the read_acq_signal.
Merge branch 'sam_alt' into sam_alt_snap
Implemented most of alt cache snapshotting, seemingly.  Now the page cache needs transaction-less read acqs.
Merge branch 'sam_alt' into sam_alt_snap
Merge branch 'sam_alt' into sam_alt_snap
Merge branch 'sam_alt_snap' of github.com:rethinkdb/rethinkdb into sam_alt_snap
Added a NORETURN to temporarily unimplemented function for the sake of clang compilation.
Merge branch 'sam_alt' into sam_alt_snap
Added page_cache_ field to current_page_acq_t in preparation for read-only acqs lacking a txn.
Made the_txn_ in the page cache always be null when access is read, made us not call add_acquirer or remove_acquirer.
Implemented read-only unattached current_page_acq_t.