Add worker pool for LDAP group lookup

[jasonodonnell]

This PR aims to make LDAP group lookup more performant by adding a worker pool for group searches. Specifically this is helpful when use_token_groups=true and we need to look up every group individually in LDAP associated with the user.

In my testing where a LDAP user is a member of 300 LDAP groups and has 300 policies mapped to those groups I see significant performance increase by using a worker pool:

Before:

$ time vault login -method=ldap username=bob password=password
Success! You are now authenticated. The token information displayed below
is already stored in the token helper. You do NOT need to run "vault login"
again. Future Vault requests will automatically use this token.

Key                    Value
---                    -----
token                  hvs.CAESIN2lzMYdtMMQ31k5MoBccnS8exljazJc6ahv40RN9sfJGh4KHGh2cy5kSGFyS3diVXNPRmIwWG5pdkw4OWV2Sjg
token_accessor         rOEMnAWD9hUWMqUL1Md5ko84
token_duration         768h
token_renewable        true
...

real	0m16.168s
user	0m0.057s
sys	0m0.027s

After:

$ time vault login -method=ldap username=bob password=password
Success! You are now authenticated. The token information displayed below
is already stored in the token helper. You do NOT need to run "vault login"
again. Future Vault requests will automatically use this token.

Key                    Value
---                    -----
token                  hvs.CAESIE311YhuFnlcsN2BYJKc0dnOnMNMnXLRBOMxpaXcD1s5Gh4KHGh2cy5iZjc0dkx5ZjdHVFBySUxiUERmTGpMSXg
token_accessor         n8QwGvA5HDXIcfB77FaT3hye
token_duration         768h
token_renewable        true
...

real	0m2.303s
user	0m0.059s
sys	0m0.029s

[github-actions]

Build Results:
All builds succeeded! ✅

[github-actions]

CI Results:
All Go tests succeeded! ✅

[mpalmi]

Good stuff! Thanks for looking into the race.

[raymonstah]

Nice improvement. Tagging @jimlambrt in case we want to port these changes to the cap/ldap package as well.

[austingebauer]

Nice improvement! Thanks @jasonodonnell 👍