Fix issue #3 and #4

[ghost]

This PR should fix #3

• Ethernet scheduler called by timer every 1ms
• function close client reworked

To test this PR please add #define DEFAULT_ETHERNET_TIMER TIM6 in lwipopts.h

The issue #4 is also fixed:

• init sequence is done only once even if begin() is called several time.

[ghost]

If this PR is validated, the register callback could be removed indeed.

[fpistm]

please @fprwi6labs could you remove the register cb. DEFAULT_ETHERNET_TIMER should also be added in the lwipot.h?

[ghost]

Please merge also:

• in core PR#136 and
• LwIP PR#2

[ghost]

Issue #4 fixed. Initialization sequence must be done only once.