[Feature] Configurable flush parameters

[lubird]

Implement two configurable parameters:

• Flush at: the number of items in a batch to upload (default 250)
• Flush interval: the frequency, in seconds, to send data to Segment (default 10)

These parameters should be available when instantiating the analytics client, and they should have separate getters and setters.

Tasks:

• Add flush at parameter
• Add flush interval parameter