introduce maxclients flag

[romange]

DragonflyDB should be able to protect itself from opening too many connections on the primary port. Otherwise, it will risk reaching max number of open files.

Dragonfly can support a high number by default, I would say 64k is ok as a default for the flag. Redis sets it as 1000: https://github.com/redis/redis/blob/6bf9b144ef722fb301c065125a6bf6cd385841bf/src/config.c#L3188

Also, we should provide a friendly warning on startup if max files ulimit is lower than maxclients - see https://github.com/redis/redis/blob/6bf9b144ef722fb301c065125a6bf6cd385841bf/src/server.c#L2278 and https://github.com/redis/redis/blob/6bf9b144ef722fb301c065125a6bf6cd385841bf/src/config.c#L2539 for example

[romange]

@ashotland @Pothulapati I suggest tracking connection count metrics in our monitoring dashboards. Also, pls create a follow-up issue - we should configure dragonfly by setting this TBD flag according to the instance memory.

[ashotland]

Connection count is already part of the dashboard.

Created controlplane issue.

[adiholden]

@royjacobson Note: We will need to support setting this flag from config set command