Adds a new sync.Mutex field to the webClient struct, rather than
using the general LocalBackend mutex. Since webClientGetOrInit
(previously WebClientInit) gets called on every connection, we
want to avoid holding the lock on LocalBackend just to check if
the server is initialized.
Moves all web_client.go funcs over to using the webClient.mu field.
Updates tailscale/corp#14335
Signed-off-by: Sonia Appasamy <sonia@tailscale.com>
055394f3be