Safe mode and name node startup procedures

This is a proposal to improve DFS cluster startup process.
The data node startup procedures were described and implemented in HADOOP-124.
I'm trying to extend them to the name node here.
The main idea is to introduce safe mode, which can be entered manually for administration
purposes, or automatically when a configurable threshold of active data nodes is breached,
or at startup when the node stays in safe mode until the minimal limit of active
nodes is reached.

This are high level requirements intended to improve the name node and cluster reliability.
= The name node safe mode means that the name node is not changing the state of the
file system. Meta data is read-only, and block replication / removal is not taking place.
= In safe mode the name node accepts data node registrations and
processes their block reports.
= The name node always starts in safe mode and stays safe until the majority
(a configurable parameter: safemode.threshold) of data nodes (or blocks?)
is reported.
= The name node can also fall into safe mode when the number of non-active
(heartbeats stopped coming in) data nodes becomes critical.
= The startup "silent period", when the name node is in safe mode and is
not issuing any block requests to the data nodes, is initially set to a
configurable value safemode.timeout.increment. By the end of the timeout
the name node checks the safemode.threshold and decides whether to switch
to the normal mode or to stay in safe. If the normal mode criteria is not
met, then the silent period is extended by incrementing the safemode timeout.
= The name node stays in safe mode not longer than a configurable value of
safemode.timeout.max, in which case it logs missing data nodes and shuts
itself down.
= When the name node switches to normal mode it checks whether all required
data nodes have actually registered, based on the list of active data storages
from the last session. Then it logs missing nodes, if any, and starts
replicating and/or deleting blocks as required.
= A historical list of data storages (nodes) ever registered with the cluster is
persistently stored in the image and log files. The list is used in two ways:
a) at startup to verify whether all nodes have registered, and to report
missing nodes;
b) at runtime if a data node registers with a new storage id the
name node verifies that no new blocks are reported from that storage,
which would prevent us from accidentally connecting data nodes from a
different cluster.
= The name node should have an option to run in safe mode. Starting with
that option would mean it never leaves safe mode.
This is useful for testing the cluster.
= Data nodes that can not connect to the name node for a long time (configurable)
should shut down themselves.