Fix ErrNoConnections from defaultConnectionPicker.Remove code

[glutamatt]

i use this driver to interact with an apache Cassandra cluster : the defaultConnPicker is used

the application queries eventually fail with the error ErrNoConnections, without any recovery

all connPicker connections eventually are set to nil by handling of sporadic network errors over time:

• Conn.closeWithError (code)
• hostConnPool.HandleError (code)
• defaultConnPicker.Remove (code)

the query executor loop over nil connections , and the recovery ( pool filling ) does not occur because the pool is full of connections

the old implementation of hostPool, before connPicker , actually remove the failed connection and shorten the slice of connections

i propose the patch to the upstream (i already patched the fork i use in my production code)

[dkropachev]

i use this driver to interact with an apache Cassandra cluster : the defaultConnPicker is used

the application queries eventually fail with the error ErrNoConnections, without any recovery

all connPicker connections eventually are set to nil by handling of sporadic network errors over time:

• Conn.closeWithError (code)
• hostConnPool.HandleError (code)
• defaultConnPicker.Remove (code)

the query executor loop over nil connections , and the recovery ( pool filling ) does not occur because the pool is full of connections

the old implementation of hostPool, before connPicker , actually remove the failed connection and shorten the slice of connections

i propose the patch to the upstream (i already patched the fork i use in my production code)

Thanks @glutamatt for your contribution.