remote-ext: improve state download performance on slow connections

[liamaharon]

Introduction

I experienced an apparent stall downloading state from https://rococo-try-runtime-node.parity-chains.parity.io:443 which was having networking difficulties only responding to my JSONRPC requests with 50-200KB/s of bandwidth.

This PR fixes the issue causing the stall, and generally improves performance remote-ext when it downloads state by greatly reducing the chances of a timeout occuring.

Description

Introduces a new REQUEST_DURATION_TARGET constant and modifies get_storage_data_dynamic_batch_size to

1. Increase or decrease the batch size of the next request depending on whether the elapsed time of the last request was gt or lt the target
2. Reset the batch size to 1 if the request times out

This fixes an issue on slow connections that can otherwise cause multiple timeouts and a stalled download:

1. The batch size increases rapidly as remote-ext downloads keys with small associated storage values
2. remote-ext tries to process a large series of subsequent keys all with extremely large associated storage values (Rococo has a series of keys 1-5MB large)
3. The huge storage values download for 5 minutes until the request times out
4. The partially downloaded keys are thrown out and remote-ext tries again with a smaller batch size, but the batch size is still far too large and takes 5 minutes to be reduced again
5. The download will be essentially stalled for many hours while the above step cycles

After this PR, the request size will

1. Not grow as large to begin with, as it is regulated downwards as the request duration exceeds the target
2. Drop immediately to 1 if the request times out. A timeout indicates the keys next in line to download have extremely large storage values compared to previously downloaded keys, and we need to reset the batch size to figure out what our new ideal batch size is. By not resetting down to 1, we risk the next request timing out again.