Batch send_after/3 calls

[aboroska]

Starting a timer for each item in the cache can be resource intensive and might even reach the capabilities of the VM with a high enough number of items.

Add an option to batch the send_after/3 calls per TTL interval by buffering them for a while in the gen_server's state and sent in one batch per TTL interval. The configuration variable is called: buffer_limit and denotes the maximum length of this internal buffer.

A regular tick event is also added to make sure that the TTL timers are started even when the number of new items do not fill the buffer in a given tick interval set by the configuration variable: expiry_tick_ms.

To keep the implementation simple all the buffered TTL intervals are emptied at each tick.

By not setting the new configuration variable buffer_limit the behaviour remains as it was before: an expiration timer is started for each new item when expriry set other than infinity.

[aboroska]

This change would make simple_cache less "simple". I argue that it is still simple enough, especially compared to other implementations, though I am not offended if it is decided to not fit in the "simple" category.