Allow configuration of r2d2 connection pool

[goyal-anshul]

We came across a scenario in our app where we needed to build r2d2::Pool with custom configuration settings, specifically setting min_idle and idle_timeout. Currently we are using memcache::Client::with_pool_size and there seem to be no way to configure settings for r2d2::Pool.

We found memcache::Client::with_pool where we can pass custom connection manager pool but it does not take an array so I can only pass a single pool for a single memcache node.

It would be nice to either allow passing configuration for r2d2 via with_pool_size function or add a new function that will take a vector of connection manager pool as shown below:

pub fn with_pools(pools: Vec<Pool<ConnectionManager>>) -> Result<Self, MemcacheError> {
        Ok(Client {
            connections: pools,
            hash_function: default_hash_function,
        })
    }

To workaround this limitation, we have patched the crate in our repo and applying some configuration to r2d2 in with_pool_size function. It would be great if this is available in the crate itself.

[aisk]

Hi @goyal-anshul Thank you for reporting this! I added your with_pools implementation in #150.

[aisk]

Shipped in v0.18.0.