Introduce properties for pool configurations

[lasanthaS]

Purpose

Introduce properties for pool configurations in HTTP sink.

Goals

The following properties were introduced to HTTP sink for configuring connection pools.

Property	Description	Default Value
connection.pool.count	Number of connection pools that need to be created for the particular client.	0
max.pool.active.connections	Maximum possible number of active connection per pool for the client.	-1 (Unlimited)
min.pool.idle.connections	Minimum allowed number of idle connections that can be existed in a pool of the client.	0
max.pool.idle.connections	Maximum number of idle connections that can be existed in a pool of the client.	100
min.evictable.idle.time	Minimum amount of time (in milliseconds) a connection may sit idle in the pool before it is eligible for eviction.	300000 ms
time.between.eviction.runs	Time between two eviction operations (in milliseconds) on the connection pool.	30000 ms
max.wait.time	The maximum number of milliseconds that the pool will wait (when there are no available connections) for a connection to be returned.	60000 ms
test.on.borrow	The indication of whether objects will be validated before being borrowed from the pool. If the object validation is failed, it will be dropped from the pool, and will attempt to borrow another.	true
test.while.idle	The indication of whether objects will be validated by the idle object evictor (if any). If the object validation is failed, it will be dropped from the pool.	true
exhausted.action	Action which should be taken when the maximum number of active connections are being used. This action is indicated as an integer. Possible action are as following. 0 - Fail the request when the pool is exhausted 1 - Block the request when the pool is exhausted, until a connection returns to the pool. 2 - Grow the connection pool size when it's exhausted.	1 (Block when exhausted)

Security checks

• Followed secure coding standards in http://wso2.com/technical-reports/wso2-secure-engineering-guidelines? yes
• Ran FindSecurityBugs plugin and verified report? yes
• Confirmed that this PR doesn't commit any keys, passwords, tokens, usernames, or other secrets? yes