[vitessdriver|vtadmin] Support Ping in vitessdriver, use in vtadmin to healthcheck connections during Dial

[ajm188]

Description

This PR fixes a longstanding issue in vtadmin, where if vtadmin had Dial-ed a cluster, obtained a connection to a vtgate, and that vtgate was torn down, that cluster would become unusable without restarting vtadmin, and we would see the following, forever:

W0317 12:23:29.496680       7 clientconn.go:1191] grpc: addrConn.createTransport failed to connect to {${somehostname} 0  <nil>}. Err :connection error: desc = "transport: Error while dialing dial tcp ${someaddr}: connect: connection refused". Reconnecting...

To fix this, I've introduced the following:

• An implementation of the database/sq/driver.Pinger interface in vitessdriver, which just issues a select 1 query.
• A call to Ping during VTGateProxy.Dial in the case where the proxy has a non-nil connection. We set a configurable timeout here, to avoid blocking forever, and if the call fails we attempt to close the connection and then fall through to the "discover new vtgate and dial it" phase of the Dial method. If the ping check succeeds, we know we can just keep using that connection.

Related Issue(s)

Checklist

• Should this PR be backported? no
• Tests were added or are not required n/a
• Documentation was added or is not required

Deployment Notes

Impacted Areas in Vitess

Components that this PR will affect:

• Query Serving (maybe?)
• VReplication
• Cluster Management
• Build/CI
• VTAdmin

[doeg]

A good suggestion, turns out!

[doeg]

Thank you!

[ajm188]

Looks like the onlineddl_devert test is having trouble downloading a dependency from mongodb:

Fetched 4582 kB in 5s (876 kB/s)
Reading package lists...
E: Failed to fetch https://repo.mongodb.org/apt/ubuntu/dists/focal/mongodb-org/4.4/multiverse/binary-arm64/Packages.gz  File has unexpected size (6835 != 6951). Mirror sync in progress? [IP: 13.32.207.84 443]
   Hashes of expected file:
    - Filesize:6951 [weak]
    - SHA512:4053c0cdc0bc113a998eacc500646779caff4d678a0739a8ff26d7c43775191062122d34220a11d9f5a5ddd6dbc407c55a1509142ee0ba10faae3a51c7017104
    - SHA256:b4ff636e06c00ebe0fca9ffa321e3574c4e4a5a075b542d7cb9690020b90a0fc
    - SHA1:22f631d805a9cc118dae094ddbd71300ce7f13b2 [weak]
    - MD5Sum:8e0a7a78501118d1e34a7bbb922d811c [weak]
   Release file created at: Wed, 17 Mar 2021 17:14:18 +0000
E: Some index files failed to download. They have been ignored, or old ones used instead.
Error: Process completed with exit code 100.

[deepthi]

LGTM