improve visibility into the bootstrapping status of a tablet server

Today, the best way to understand the happenings of a tserver startup is to look at the /tablets page in the tserver's web UI and watch the number of RUNNING tablets gradually but hopefully increase.

It might be helpful to know about some other things, like:

• progress in initializing the block manager
• what tablets are currently being bootstrapped
• the size of the WALs for tablets that need bootstrapping, since this may indicate how much work there is to bootstrap
• the number of rowsets contained by the tablets that need bootstrapping, since this may indicate how much work there is to bootstrap

Also, in a similar way that we label recovering tservers as "recovering", we may want to consider softer language like "initializing" or "bootstrapping" for tablets that have any replicas in the INITIALIZING state.