sql: initialize with a limited number of table leases

[vivekmenezes]

This will prevent initial requests hitting a server from
blocking on lease acquisition.

related to #23510

Release note: Fixed slowness caused by table lease acquisition
at startup.

[cockroach-teamcity]

This change is 

[vivekmenezes]

@asubiotto perhaps you have some thoughts on how I might fix the TODO on this PR. I have this initialization method that is running some internal SQL on a system table, and would like to have it reach completion before a node receives traffic. But I worry that we have a chicken and egg situation where if all nodes wait to serve traffic they will not be able to serve the internal SQL request.

[asubiotto]

The only thing I can think of is to add something to

cockroach/pkg/server/status.go

Line 577 in b33f4fb

serveMode := s.admin.server.serveMode.get()

that returns that the server is ready to accept client traffic only when this lease acquisition has been performed. Although this won't work if the client is not observing the /health?ready=1 endpoint (but maybe that's the expectation?) @bdarnell or @a-robinson might have a better idea.

[dt]

I think it is okay to accept traffic before this runs -- blocking and queuing connections on it will mean they see just as much, if not more, latency than just letting them hit a cold cache.

[bdarnell]

I agree with @dt, just let the node start up and let anything that comes in before the lease acquisition finishes block to get the lease.